avcodec/dvbsubenc: fix 256 color rle decoding

The end of 8-bit/pixel_code_string() is signalled with two zero bytes, so we
should read those from the buffer instead of one. We keep supporting though our
own buggy encoder which only put one zero byte there.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavcodec/dvbsubdec.c

@@ -660,6 +660,10 @@ static int dvbsub_read_8bit_string(AVCodecContext *avctx,
     if (bytestream2_get_byte(gb))
         av_log(avctx, AV_LOG_ERROR, "line overflow\n");
 
+    /* Workaround our own buggy encoder which only put one zero at the end */
+    if (!bytestream2_peek_byte(gb))
+        bytestream2_get_byte(gb);
+
     *srcbuf += bytestream2_tell(gb);
     return pixels_read;
 }