avcodec: fix aac/ac3 parser bitstream buffer size

Buffers containing copies of the AAC and AC3 header bits were not padded
before parsing, violating init_get_bits() buffer padding requirement,
leading to potential buffer read overflows.
This change adds FF_INPUT_BUFFER_PADDING_SIZE bytes to the bit buffer
for parsing the header in each of aac_parser.c and ac3_parser.c.

Based on patch by: Matt Wolenetz <wolenetz@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>