configure: Only build ffserver if SA_RESAMPLE is available.

QNX does not define SA_RESAMPLE (because it is not implemented).
Fixes ticket #2780.

Reviewed-by: Nicolas George
g726: Do not try to decode more than one channel.

Ask for a sample instead.

Reviewed-by: Derek Buitenhuis
rmdec: Forward error messages from rm_assemble_video_frame() to the caller.

Fixes the cause of a null pointer dereference on oom
described in ticket #2724.