ffserver: allocate AVStream's internal too

Avoids segfault at init_muxer() (mux.c) due to a
null pointer dereference on the recently
introduced AVStream->internal

Fixes: #5059 (https://trac.ffmpeg.org/ticket/5059)

Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>
