Crash fix for when a wxNullBitmap is passed in as the bitmap to use.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58782 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/cocoa/bmpbuttn.mm

@@ -47,7 +47,9 @@ wxWidgetImplType* wxWidgetImpl::CreateBitmapButton( wxWindowMac* wxpeer,
             [v setBezelStyle:NSRegularSquareBezelStyle];
     }
     
+    if (bitmap.Ok())
         [v setImage:bitmap.GetNSImage() ];
+    
     [v setButtonType:NSMomentaryPushInButton];
     wxWidgetCocoaImpl* c = new wxWidgetCocoaImpl( wxpeer, v );
     return c;