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>

libavcodec/aac_parser.c

@@ -34,7 +34,7 @@ static int aac_sync(uint64_t state, AACAC3ParseContext *hdr_info,
     int size;
     union {
         uint64_t u64;
-        uint8_t  u8[8];
+        uint8_t  u8[8 + FF_INPUT_BUFFER_PADDING_SIZE];
     } tmp;
 
     tmp.u64 = av_be2ne64(state);

libavcodec/ac3_parser.c

@@ -166,7 +166,7 @@ static int ac3_sync(uint64_t state, AACAC3ParseContext *hdr_info,
     int err;
     union {
         uint64_t u64;
-        uint8_t  u8[8];
+        uint8_t  u8[8 + FF_INPUT_BUFFER_PADDING_SIZE];
     } tmp = { av_be2ne64(state) };
     AC3HeaderInfo hdr, *phdr = &hdr;
     GetBitContext gbc;