avformat/webmdashenc: Remove possibility of infinite loop

The WebM DASH manifest muxer uses a loop to parse the adaptation_sets
string (which is given by the user and governs which AVStreams are
mapped to what adaptation set) and the very beginning of this loop is
"if (*p == ' ') continue;". This of course leads to an infinite loop if
the condition is true. It is true if e.g. the string begins with ' ' or
if there are more than one ' ' between different adaptation set groups.

To fix this, the parsing process has been modified to consume the space
if it is at a place where it can legitimately occur, i.e. when a new
adaptation set group is expected. The latter restriction implies that an
error is returned if a space exists where none is allowed to exist.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/webmdashenc.c

@@ -438,9 +438,10 @@ static int parse_adaptation_sets(AVFormatContext *s)
     // 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)) {
-        if (*p == ' ')
+        if (state == new_set && *p == ' ') {
+            p++;
             continue;
-        else if (state == new_set && !strncmp(p, "id=", 3)) {
+        } 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)