Bug 570451 - Avoid adding unneeded lines to image; r=joe sr=vlad

modules/libpr0n/decoders/png/nsPNGDecoder.cpp

@@ -776,6 +776,9 @@ row_callback(png_structp png_ptr, png_bytep new_row,
   if (decoder->mFrameIsHidden)
     return;
 
+  if (row_num >= decoder->mFrameRect.height)
+    return;
+
   if (new_row) {
     PRInt32 width = decoder->mFrameRect.width;
     PRUint32 iwidth = decoder->mFrameRect.width;