avcodec/h263dec: Return the correct error code in explode mode

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h263dec.c

@@ -713,7 +713,7 @@ frame_end:
     }
 
     if (slice_ret < 0 && (avctx->err_recognition & AV_EF_EXPLODE))
-        return ret;
+        return slice_ret;
     else
         return get_consumed_bytes(s, buf_size);
 }