ffmpeg: Check read_ffserver_streams() return value

The function avformat_alloc_context() will return a NULL pointer on
failure. However, in function read_ffserver_streams(), its return value
is not validated and the subsequent dereference may result in a bad
memory access bug. Check its return value against NULL and avoid
potential NULL dereference.

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