avformat/webmdashenc: Be more strict when parsing stream indices

avformat/webmdashenc: Be more strict when parsing stream indices

The syntax of the adaptation_sets string by which the user determines
the mapping of AVStreams to adaptation sets is
"id=x,streams=a,b,c id=y,streams=d,e" (means: the streams with the
indices a, b and c belong to the adaptation set with id x). Yet there
was no check for whether these indices were actual numbers and if there
is a number whether it really extends to the next ',', ' ' or to the
end of the string or not. This commit adds a check for this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>