libavformat/rtpenc_mpegts: check avformat_new_stream() return value

The function avformat_new_stream() returns a NULL pointer on failure.
However, in function rtp_mpegts_write_header(), its return value is not
validated before it is dereferenced. Check the return value against NULL
to avoid potential NULL dereference.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
