avidec: Return correct error code from avi_sync() in case of IO error.

Found-by: Tanami, Ohad
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avidec.c

@@ -1001,6 +1001,8 @@ start_sync:
         }
     }
 
+    if(pb->error)
+        return pb->error;
     return AVERROR_EOF;
 }