avcodec/ac3enc: Use actual size of buffer in init_put_bits()

avcodec/ac3enc: Use actual size of buffer in init_put_bits()

Since the very beginning (since de6d9b6404bfd1c589799142da5a95428f146edd)
the AC-3 encoder used AC3_MAX_CODED_FRAME_SIZE (namely 3840) for the
size of the output buffer (without any check at all).
This causes problems when encoding EAC-3 for which the maximum is too small,
smaller than the actual size of the buffer: One can run into asserts used
by the PutBits API. Ticket #8513 is about such a case and this commit
fixes it by using the real size of the buffer.

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