Be more defensive in getting max texture slot count.

src/Magnum/Implementation/TextureState.cpp

@@ -34,7 +34,7 @@
 
 namespace Magnum { namespace Implementation {
 
-TextureState::TextureState(Context& context, std::vector<std::string>& extensions): maxMaxAnisotropy(0.0f), currentLayer(0)
+TextureState::TextureState(Context& context, std::vector<std::string>& extensions): maxLayers(0), maxMaxAnisotropy(0.0f), currentLayer(0)
     #ifndef MAGNUM_TARGET_GLES
     , maxColorSamples(0), maxDepthSamples(0), maxIntegerSamples(0), bufferOffsetAlignment(0)
     #endif
@@ -160,6 +160,7 @@ TextureState::TextureState(Context& context, std::vector<std::string>& extension
 
     /* Resize bindings array to hold all possible layers */
     glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &maxLayers);
+    CORRADE_INTERNAL_ASSERT(maxLayers > 0);
     bindings.resize(maxLayers);
 }