Check that the -P option was specified

-P <theora|webm> is to inform how to encode the VOBs. It must be
specified. Previously if you did something like

    encode_vobs *.vob

you would get the following

    15:40:30: Using profile: (null)
    15:40:30: Using 3 cores
    Segmentation fault

due to the profile not being specified.

Fix this by making sure that the profile has been set before attempting
to continue.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>