Check for having valid image size in wxGIFHandler saving code.

At least the width must be non-0 as we allocate a width-sized array below and
0 height is probably problematic -- and definitely not useful -- as well, so
just bail out if either of them is 0.

src/common/imaggif.cpp

@@ -218,6 +218,8 @@ bool wxGIFHandler::DoSaveFile(const wxImage& image, wxOutputStream *stream,
 
     int width = image.GetWidth();
     int height = image.GetHeight();
+    wxCHECK_MSG( width && height, false, wxS("can't save 0-sized file") );
+
     int width_even = width + ((width % 2) ? 1 : 0);
 
     if (first)