codecparsers: mpeg2: don't mess the StartCode only packets

It is completely legal to have packets with zero sizes.
Zero-sized packet indicates header with only Start Code.
One eg: is user data packet. The patch allows having
GstMpegVideoPacket with zero sizes.

https://bugzilla.gnome.org/show_bug.cgi?id=796477
diff --git a/gst-libs/gst/codecparsers/gstmpegvideoparser.c b/gst-libs/gst/codecparsers/gstmpegvideoparser.c
index 5d0e6a7..408ae03 100644
--- a/gst-libs/gst/codecparsers/gstmpegvideoparser.c
+++ b/gst-libs/gst/codecparsers/gstmpegvideoparser.c
@@ -264,7 +264,7 @@
   size -= off + 4;
   off = scan_for_start_codes (&br, 0, size);
 
-  if (off > 0)
+  if (off >= 0)
     packet->size = off;
 
   return TRUE;