mpeg12: terminate mpeg2_video_profiles arrays

Make av_get_profile_name() return NULL if no profile is detected.

Fix trac issue #130, fix crash reading file tek3.m2v.
(cherry picked from commit e5d80c7b2d893422e2e60a97e08bfc48ca1684e6)

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
