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_timestamp(AVIOContext *pb, int64_t ts) {
    avio_wb24(pb, ts & 0xFFFFFF);
    avio_w8(pb, (ts >> 24) & 0x7F);
}

static void put_timestamp(AVIOContext *pb, int64_t ts) {
    avio_wb24(pb, ts & 0xFFFFFF);
    avio_w8(pb, (ts >> 24) & 0x7F);
}
