avformat/rtmppkt: Convert ff_amf_tag_size() to bytestream2

      Fixes: out of array accesses
Fixes: crash-9238fa9e8d4fde3beda1f279626f53812cb001cb-SEGV

Found-by: JunDong Xie of Ant-financial Light-Year Security Lab
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>