Platform: ensure GLX configs are freed even in case of an error.

I thought this would fix a leak reported by ASan when running on Mesa 25
llvmpipe, but it didn't. Given that no leak is reported when *not*
running on llvmpipe, I assume this is llvmpipe's fault.

src/Magnum/Platform/WindowlessGlxApplication.cpp

@@ -27,6 +27,7 @@
 
 #include "WindowlessGlxApplication.h"
 
+#include <Corrade/Containers/ScopeGuard.h>
 #include <Corrade/Containers/StringView.h>
 #include <Corrade/Utility/Assert.h>
 #include <Corrade/Utility/Debug.h>
@@ -100,6 +101,7 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
         Error() << "Platform::WindowlessGlxContext: no supported framebuffer configuration found";
         return;
     }
+    Containers::ScopeGuard freeConfigs{configs, XFree};
 
     /* Create pbuffer */
     constexpr static const int pbufferAttributes[] = {
@@ -266,8 +268,6 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
     static_cast<void>(magnumContext);
     #endif
 
-    XFree(configs);
-
     if(!_context) {
         Error e;
         e << "Platform::WindowlessGlxContext: cannot create context";