avidec: return 0, not packet size from read_packet().

(cherry picked from commit eeade678f0)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/avidec.c

@@ -990,7 +990,7 @@ resync:
             ast->packet_size= 0;
         }
 
-        return size;
+        return 0;
     }
 
     memset(d, -1, sizeof(int)*8);