avformat/flvenc: Check audio packet size

Fixes: Assertion failure
Fixes: assert_flvenc.c:941_1.swf

Found-by: #CHEN HONGXU# <HCHEN017@e.ntu.edu.sg>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static void put_amf_bool(AVIOContext *pb, int b)
{
    avio_w8(pb, AMF_DATA_TYPE_BOOL);
    avio_w8(pb, !!b);
}

static void put_amf_bool(AVIOContext *pb, int b)
{
    avio_w8(pb, AMF_DATA_TYPE_BOOL);
    avio_w8(pb, !!b);
}
