The code currently presumes that a return value of AVERROR(ENOMEM)
implies that ac3hdr could not be allocated, so it need not be freed.
Yet any avpriv_ac3_parse_header() might allocate more than the
AC3HeaderInfo internally (it doesn't currently), so simply free
it unconditionally.

Fixes Coverity issues #1492870 and #1492868.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>