Fix creating wxGLCanvas without any attributes in wxMSW

This was broken by the changes of
f0e67ed517, see #16402.

Just check that we do have the attributes before examining them.

(cherry picked from commit 74421a7f35)

src/msw/glcanvas.cpp

@@ -321,6 +321,8 @@ bool wxGLCanvas::Create(wxWindow *parent,
         glVersionMinor = 0;
 
     // Check for a core profile request
+    if ( attribList )
+    {
         for ( int i = 0; attribList[i]; )
         {
             switch ( attribList[i++] )
@@ -342,6 +344,7 @@ bool wxGLCanvas::Create(wxWindow *parent,
                     break;
             }
         }
+    }
 
     if ( useGLCoreProfile )
     {