avformat/webmdashenc: Check id in adaption_sets

Fixes: out of array access

Found-by: Wenxiang Qian
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b687b549aa)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/webmdashenc.c

@@ -439,6 +439,7 @@ static int parse_adaptation_sets(AVFormatContext *s)
             continue;
         else if (state == new_set && !strncmp(p, "id=", 3)) {
             void *mem = av_realloc(w->as, sizeof(*w->as) * (w->nb_as + 1));
+            const char *comma;
             if (mem == NULL)
                 return AVERROR(ENOMEM);
             w->as = mem;
@@ -447,6 +448,11 @@ static int parse_adaptation_sets(AVFormatContext *s)
             w->as[w->nb_as - 1].streams = NULL;
             p += 3; // consume "id="
             q = w->as[w->nb_as - 1].id;
+            comma = strchr(p, ',');
+            if (!comma || comma - p >= sizeof(w->as[w->nb_as - 1].id)) {
+                av_log(s, AV_LOG_ERROR, "'id' in 'adaptation_sets' is malformed.\n");
+                return AVERROR(EINVAL);
+            }
             while (*p != ',') *q++ = *p++;
             *q = 0;
             p++;