Replaced comment with assertion.

src/wgl_context.c

@@ -122,12 +122,13 @@ static void initWGLExtensions(_GLFWwindow* window)
 }
 
 // Returns the specified attribute of the specified pixel format
-// NOTE: Do not call this unless we have found WGL_ARB_pixel_format
 //
 static int getPixelFormatAttrib(_GLFWwindow* window, int pixelFormat, int attrib)
 {
     int value = 0;
 
+    assert(window->wgl.ARB_pixel_format);
+
     if (!window->wgl.GetPixelFormatAttribivARB(window->wgl.dc,
                                                pixelFormat,
                                                0, 1, &attrib, &value))