Platform: check for non-null GL_VENDOR before comparing it.

Otherwise, when context creation fails *really bad*, this crashes
somewhere deep in __strncmp_sse42 or so.

doc/changelog.dox

@@ -432,6 +432,12 @@ See also:
     [mosra/magnum-bootstrap#15](https://github.com/mosra/magnum-bootstrap/pull/15))
 -   @ref Trade::TgaImporter "TgaImporter" now properly handles files with too
     short data (see [mosra/magnum#359](https://github.com/mosra/magnum/issues/359))
+-   When context creation fails *really bad* with
+    @ref Platform::Sdl2Application, @ref Platform::GlfwApplication,
+    @ref Platform::WindowlessGlxApplication or
+    @ref Platform::WindowlessWglApplication, driver detection for the
+    @cpp "no-forward-compatible-core-context" @ce now properly handles
+    @fn_gl{GetString} returning a @cpp nullptr @ce
 
 @subsection changelog-latest-docs Documentation
 

src/Magnum/Platform/GlfwApplication.cpp

@@ -401,9 +401,12 @@ bool GlfwApplication::tryCreate(const Configuration& configuration, const GLConf
     #endif
     if(glConfiguration.version() == GL::Version::None && (!_window
         #ifndef CORRADE_TARGET_APPLE
-        /* Sorry about the UGLY code, HOPEFULLY THERE WON'T BE MORE WORKAROUNDS */
+        /* If context creation fails *really bad*, glGetString() may actually
+           return nullptr. Check for that to avoid crashes deep inside
+           strncmp(). Sorry about the UGLY code, HOPEFULLY THERE WON'T BE MORE
+           WORKAROUNDS */
         || (vendorString = reinterpret_cast<const char*>(glGetString(GL_VENDOR)),
-        (std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
+        vendorString && (std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
          #ifdef CORRADE_TARGET_WINDOWS
          std::strncmp(vendorString, intelVendorString, sizeof(intelVendorString)) == 0 ||
          #endif

src/Magnum/Platform/Sdl2Application.cpp

@@ -410,9 +410,12 @@ bool Sdl2Application::tryCreate(const Configuration& configuration, const GLConf
     #endif
     if(glConfiguration.version() == GL::Version::None && (!_glContext
         #ifndef CORRADE_TARGET_APPLE
-        /* Sorry about the UGLY code, HOPEFULLY THERE WON'T BE MORE WORKAROUNDS */
+        /* If context creation fails *really bad*, glGetString() may actually
+           return nullptr. Check for that to avoid crashes deep inside
+           strncmp(). Sorry about the UGLY code, HOPEFULLY THERE WON'T BE MORE
+           WORKAROUNDS */
         || (vendorString = reinterpret_cast<const char*>(glGetString(GL_VENDOR)),
-        (std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
+        vendorString && (std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
          #ifdef CORRADE_TARGET_WINDOWS
          std::strncmp(vendorString, intelVendorString, sizeof(intelVendorString)) == 0 ||
          #endif

src/Magnum/Platform/WindowlessGlxApplication.cpp

@@ -123,7 +123,10 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
         constexpr static const char nvidiaVendorString[] = "NVIDIA Corporation";
         constexpr static const char amdVendorString[] = "ATI Technologies Inc.";
         const char* const vendorString = reinterpret_cast<const char*>(glGetString(GL_VENDOR));
-        if((std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
+        /* If context creation fails *really bad*, glGetString() may actually
+           return nullptr. Check for that to avoid crashes deep inside
+           strncmp() */
+        if(vendorString && (std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
             std::strncmp(vendorString, amdVendorString, sizeof(amdVendorString)) == 0) &&
             (!magnumContext || !magnumContext->isDriverWorkaroundDisabled("no-forward-compatible-core-context")))
         {

src/Magnum/Platform/WindowlessWglApplication.cpp

@@ -176,7 +176,10 @@ WindowlessWglContext::WindowlessWglContext(const Configuration& configuration, G
         constexpr static const char intelVendorString[] = "Intel";
         constexpr static const char amdVendorString[] = "ATI Technologies Inc.";
         const char* const vendorString = reinterpret_cast<const char*>(glGetString(GL_VENDOR));
-        if((std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
+        /* If context creation fails *really bad*, glGetString() may actually
+           return nullptr. Check for that to avoid crashes deep inside
+           strncmp() */
+        if(vendorString && (std::strncmp(vendorString, nvidiaVendorString, sizeof(nvidiaVendorString)) == 0 ||
             std::strncmp(vendorString, intelVendorString, sizeof(intelVendorString)) == 0 ||
             std::strncmp(vendorString, amdVendorString, sizeof(amdVendorString)) == 0) &&
             (!magnumContext || !magnumContext->isDriverWorkaroundDisabled("no-forward-compatible-core-context")))