Fix assert when creating wxBitmapButton without a valid bitmap in wxMSW.

wxBitmapButton can be created without a valid bitmap if SetBitmapLabel() is
called later, so don't call SetBitmapLabel() from the constructor if no bitmap
was provided.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/bmpbuttn.cpp

@@ -80,7 +80,8 @@ bool wxBitmapButton::Create(wxWindow *parent,
                                      validator, name) )
         return false;
 
-    SetBitmapLabel(bitmap);
+    if ( bitmap.IsOk() )
+        SetBitmapLabel(bitmap);
 
     if ( !size.IsFullySpecified() )
     {