Make h264 decoder conform to requirements of mpegvideo's MPV_frame_start.

Make h264 decoder conform to requirements of mpegvideo's MPV_frame_start.
Namely, that it should not be called if you are starting to decode a B
frame without any reference pictures.
Prevents an endless allocation cycle in MPV_frame_start that will end in
picture buffer overflow and abort.
Fixes roundup issue 216.

Originally committed as revision 11214 to svn://svn.ffmpeg.org/ffmpeg/trunk