Fix crash in graphics benchmark

Set the GL context pointer deleted in the frame dtor to NULL to ensure we
don't crash on exit when not using OpenGL.

tests/benchmarks/graphics.cpp

@@ -103,6 +103,7 @@ public:
 
 #if wxUSE_GLCANVAS
         m_glCanvas = NULL;
+        m_glContext = NULL;
 
         if ( opts.useGL )
         {