asfdec: handle the case when the stream index has an invalid value better

The demuxer returned INVALIDDATA and failed to demux the remaining data
when an invalid stream index was read, now it just skips the asf packet
for the stream with an invalid stream index and continues demuxing.

Reported-By: Hendrik Leppkes

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
