GL: properly initialize XFB binding in the state tracker.

Discovered by PVS-Studio, many thanks to @alexesDev for collecting the
report.

src/Magnum/GL/Implementation/TransformFeedbackState.cpp

@@ -7,10 +7,11 @@
 
 namespace Magnum { namespace GL { namespace Implementation {
 
-TransformFeedbackState::TransformFeedbackState(Context& context, std::vector<std::string>& extensions): maxInterleavedComponents{0}, maxSeparateAttributes{0}, maxSeparateComponents{0}
+TransformFeedbackState::TransformFeedbackState(Context& context, std::vector<std::string>& extensions): maxInterleavedComponents{0}, maxSeparateAttributes{0}, maxSeparateComponents{0},
     #ifndef MAGNUM_TARGET_GLES
-    , maxBuffers{0}, maxVertexStreams{0}
+    maxBuffers{0}, maxVertexStreams{0},
     #endif
+    binding{0}
 {
     #ifndef MAGNUM_TARGET_GLES
     if(context.isExtensionSupported<Extensions::ARB::direct_state_access>()) {