don't attempt to create 0*0 bitmaps in GetBuffer() (patch 1899643)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@52152 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/dcbufcmn.cpp

@@ -53,6 +53,14 @@ public:
                     h > ms_buffer->GetHeight() )
         {
             delete ms_buffer;
+
+            // we must always return a valid bitmap but creating a bitmap of
+            // size 0 would fail, so create a 1*1 bitmap in this case
+            if ( !w )
+                w = 1;
+            if ( !h )
+                h = 1;
+
             ms_buffer = new wxBitmap(w, h);
         }
         return ms_buffer;