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.

libavcodec/mpeg12.c

@@ -2566,6 +2566,7 @@ static const AVProfile mpeg2_video_profiles[] = {
     { FF_PROFILE_MPEG2_SIMPLE,       "Simple"             },
     { FF_PROFILE_RESERVED,           "Reserved"           },
     { FF_PROFILE_RESERVED,           "Reserved"           },
+    { FF_PROFILE_UNKNOWN },
 };