avformat/oggparsespeex: Check frames_per_packet and packet_size

The speex specification does not seem to restrict these values, thus
the limits where choosen so as to avoid multiplicative overflow

Fixes undefined behavior
Fixes: 635422.ogg

Found-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
