don't interrupt non-interruptible actions when switching dialog phases

Fixes the party member not being released after the Selfish hell trial.
DialogHandler: minor housekeeping
drop unnecessary NULL checks, fix a leak
don't call ProcessActions twice for area scripts

Without this change, ProcessActions would be called twice per tick for
area scripts. Among other problems that means that short blocking
actions, like for example SmallWait(1), are (sometimes) essentially
turned into non-blocking actions.

This is easily seen in the final scene in ToB after defeating Melissan
when all of the party members are supposed to initiate dialog. Without
this change some of the dialogs are skipped. Other scripts may break in
similar ways.
try to load mvesnd.2da silently
check for unscheduled actors via ValidTarget instead
don't allow disabled actors to initiate dialog