avformat/rtmppkt: Convert ff_amf_get_field_value() to bytestream2

Fixes: out of array accesses

Found-by: JunDong Xie of Ant-financial Light-Year Security Lab
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
void ff_amf_write_null(uint8_t **dst)
{
    bytestream_put_byte(dst, AMF_DATA_TYPE_NULL);
}

void ff_amf_write_null(uint8_t **dst)
{
    bytestream_put_byte(dst, AMF_DATA_TYPE_NULL);
}
