lavc: fix recoded subtitles end.

Text subtitles packets are not 0-terminated (and if they are,
it is handled by the recoding process since 0 is a valid
Unicode code point). The terminating 0 would overwrite the
last payload octet.

OTOH, packets must be 0-padded.

Fix a problem reported in trac ticket #2431.

libavcodec/utils.c

@@ -2185,7 +2185,7 @@ static int recode_subtitle(AVCodecContext *avctx,
         goto end;
     }
     outpkt->size -= outl;
-    outpkt->data[outpkt->size - 1] = '\0';
+    memset(outpkt->data + outpkt->size, 0, outl);
 
 end:
     if (cd != (iconv_t)-1)

tests/ref/fate/sub-charenc

-a39d7e299a8e25b4ffece0f8d64bf19e
+9615088d613931b50fd5016c7535b99a