avformat/utils: Fix confusing return value for ff_read_packet()

avformat/utils: Fix confusing return value for ff_read_packet()

Currently, ff_read_packet() sometimes forwards the return value of
AVInputFormat.read_packet() (which should be zero on success, but isn't
for all demuxers) and sometimes it overwrites this with zero.
Furthermore, it uses two variables, one for the read_packet return value
and one for other errors, which is a bit confusing; it is also
unnecessary given that the documentation explicitly states that
ff_read_packet() never returns positive values. Returning a positive
value would lead to leaks with some callers (namely asfrtp_parse_packet
and estimate_timings_from_pts). So always return zero in case of
success.

(This behaviour stems from a time before av_read_packet sanitized
the return value of read_packet at all: It was added in commit
626004690c23c981f67228ea325dde3f35193988 and was unnecessary since
88b00723906f68b7563214c30333e48888dddf78.)

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