avformat/webmdashenc: Require the 'adaptation_sets' option to be set

This seems to be non-optional, and if the muxer is run without it,
strlen() is run on NULL, causing a segfault.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/webmdashenc.c

@@ -433,6 +433,10 @@ static int parse_adaptation_sets(AVFormatContext *s)
     char *p = w->adaptation_sets;
     char *q;
     enum { new_set, parsed_id, parsing_streams } state;
+    if (!w->adaptation_sets) {
+        av_log(s, AV_LOG_ERROR, "The 'adaptation_sets' option must be set.\n");
+        return AVERROR(EINVAL);
+    }
     // syntax id=0,streams=0,1,2 id=1,streams=3,4 and so on
     state = new_set;
     while (p < w->adaptation_sets + strlen(w->adaptation_sets)) {