nut: use meaningful error values

Fix the mispresented EINVAL for EOF on partial files, among the other.
lavc: do not leak frames in avcodec_decode_audio4

Notwithstanding the return value the frame should be freed if
is not going to be used.

Reported-by: Matthew Wolenetz <wolenetz@chromium.org>
oma: return meaningful errors
oma: properly forward errors in oma_read_packet

Prevent spurios EIO on EOF.

CC:libav-stable@libav.org