sp5xdec: sanitize return value.

i is the decoded size of a recoded packet, which is larger
than the original packet. Assume that if decoding succeeded,
all the packet was used.

libavcodec/sp5xdec.c

@@ -90,7 +90,7 @@ static int sp5x_decode_frame(AVCodecContext *avctx,
 
     av_free(recoded);
 
-    return i;
+    return i < 0 ? i : avpkt->size;
 }
 
 #if CONFIG_SP5X_DECODER