Fixed Coverity reported issue.

CID 11855: Use after free (USE_AFTER_FREE)
"operator delete[](void *)" frees "buffer".

src/osgPlugins/gif/ReaderWriterGIF.cpp

@@ -548,7 +548,10 @@ GifImageStream** obj)
 
     // Delete the last allocated buffer to avoid memory leaks if we using GifImageStream
     if ( obj && *obj )
+    {
         delete [] buffer;
+	buffer = 0;
+    }
 
     delete [] rowdata;
     *width_ret = giffile->SWidth;