lavf/matroskadec: Do not use strncat() to limit copying a one-char constant.

Instead add the character to the snprintf above as suggested by Mark.

Silences a warning:
libavformat/matroskadec.c: In function 'webm_dash_manifest_cues':
libavformat/matroskadec.c:3947:13: warning: 'strncat' specified bound 1 equals source length [-Wstringop-overflow=]
             strncat(buf, ",", 1);
             ^~~~~~~~~~~~~~~~~~~~

libavformat/matroskadec.c

@@ -3936,17 +3936,14 @@ static int webm_dash_manifest_cues(AVFormatContext *s, int64_t init_range)
     strcpy(buf, "");
     for (i = 0; i < s->streams[0]->nb_index_entries; i++) {
         int ret = snprintf(buf + end, 20,
-                           "%" PRId64, s->streams[0]->index_entries[i].timestamp);
+                           "%" PRId64"%s", s->streams[0]->index_entries[i].timestamp,
+                           i != s->streams[0]->nb_index_entries - 1 ? "," : "");
         if (ret <= 0 || (ret == 20 && i ==  s->streams[0]->nb_index_entries - 1)) {
             av_log(s, AV_LOG_ERROR, "timestamp too long.\n");
             av_free(buf);
             return AVERROR_INVALIDDATA;
         }
         end += ret;
-        if (i != s->streams[0]->nb_index_entries - 1) {
-            strncat(buf, ",", 1);
-            end++;
-        }
     }
     av_dict_set(&s->streams[0]->metadata, CUE_TIMESTAMPS, buf, 0);
     av_free(buf);