Fix crash when transcoding from mpeg2

Fix crash when transcoding from mpeg2

The attached patch fixes transcoding from mpeg2 with last VLC from git.

jupiter@~/vlc> ./vlc file.ts --sout
'#transcode{vcodec=mp4v}:std{access=file,mux=ts,dst=/dev/null}' -I
dummy
[...]
[0x212d9c0] libmpeg2 decoder debug: 720x576 (display 720,576), aspect
768000, sar 64:45, 25.000 fps
[0x212d9c0] main decoder warning: can't get output picture
[0x212d9c0] libmpeg2 decoder warning: invalid picture encountered
[0x212d9c0] main decoder warning: can't get output picture
zsh: segmentation fault  ./vlc file.ts --sout  -I dummy

Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>