avidec: use actually read size instead of requested size

Fixes CVE-2012-2788
(cherry picked from commit 0af49a63c7)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavformat/avidec.c

@@ -780,7 +780,7 @@ resync:
             else
                 ast->frame_offset++;
         }
-        ast->remaining -= size;
+        ast->remaining -= err;
         if(!ast->remaining){
             avi->stream_index= -1;
             ast->packet_size= 0;