Fix wxImageList test failing under wxQt

src/qt/bitmap.cpp

@@ -601,7 +601,12 @@ void wxBitmap::QtBlendMaskWithAlpha()
     {
         AllocExclusive();
         M_PIXDATA.setMask(*M_MASK->GetHandle());
-        wxDELETE(M_MASK);
+
+        // Notice that if the mask was created from a colour that does not exist in
+        // the bitmap, setMask() will have no effect on the bitmap.  So only delete
+        // it if it has been applied successfully.
+        if ( HasAlpha() )
+            wxDELETE(M_MASK);
     }
 }