avformat/asfdec_o: Don't segfault with lots of attached pics

avformat/asfdec_o: Don't segfault with lots of attached pics

The ASF file format has a limit of 127 streams and the "asf_o" demuxer
(the ASF demuxer from Libav) has an array of pointers for a structure
called ASFStream that is allocated on demand for every stream. Attached
pictures are not streams in the sense of the ASF specification, yet the
demuxer created an ASFStream for them; and in one codepath it also
forgot to check whether the array of ASFStreams is already full. The
result is a write beyond the end of the array and a segfault lateron.

Fixing this is easy: Don't create ASFStreams for attached picture
streams.

(Other results of the current state of affairs are unnecessary allocations
(of ASFStreams structures), the misparsing of valid files (there might not
be enough ASFStreams left for the valid streams if attached pictures take
up too many); furthermore, the ASFStreams created for attached pictures all
have the stream number 0, an invalid stream number (the valid range is
1-127). This means that invalid data (packets for a stream with stream
number 0) won't get rejected lateron.)

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