This method used to return a dangling pointer to a temporary buffer, which resulted in a crash when using it, e.g. in the unit test. Fix this by keeping a QImage as a member in wxBitmapRefData, so that the pointer to its data remain valid until UngetRawData() is called. Also check that GetRawData() returns a non-null pointer in the test. Closes https://github.com/wxWidgets/wxWidgets/pull/1067