avutil/avstring: do not lose ascii characters when decoding non utf-8 with av_utf8_decode()

Fixes Ticket3363
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avstring.c

@@ -331,15 +331,15 @@ int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
     while (code & top) {
         int tmp;
         if (p >= buf_end) {
-            ret = AVERROR(EILSEQ); /* incomplete sequence */
-            goto end;
+            (*bufp) ++;
+            return AVERROR(EILSEQ); /* incomplete sequence */
         }
 
         /* we assume the byte to be in the form 10xx-xxxx */
         tmp = *p++ - 128;   /* strip leading 1 */
         if (tmp>>6) {
-            ret = AVERROR(EILSEQ);
-            goto end;
+            (*bufp) ++;
+            return AVERROR(EILSEQ);
         }
         code = (code<<6) + tmp;
         top <<= 5;