Fixes #15357

stream_demux cannot seek. This code path should never be used.

Note: to skip data explicitly, a demuxer is supposed to call
stream_Read() with a NULL buffer address.

Both functions can always fail. Even if the stream size is known,
and larger than the requested bytes, reading can fail due to lower
level errors. stream_Peek() can also fail to allocate memory.

Not checking the return value is a bug.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>