avformat/adtsenc: return value check for init_get_bits in adts_decode_extradata

As the second argument for init_get_bits (buf) can be crafted, a return value check for this function call is necessary.
'buf' is  part of  'AVPacket pkt'.
replace init_get_bits with init_get_bits8.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>