ac3: Do not clash with normal AVERROR

The parsing function return AVERROR and AAC_AC3_PARSE_ERROR values,
make sure they are not misunderstood.

(cherry picked from commit 6258d362b82934a2c27557e0984aed372d98091a)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
ac3: Clean up the error paths

(cherry picked from commit 818d1f1a3e89d35213af0bd5dc4a772713951882)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
ac3: Return proper error codes

(cherry picked from commit b1f9cdc37ff5d5b391d2cd9af737ab4e5a0fc1c0)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>