EMI: Fix crash when restoring savegames

EMI does not instantiate a registry object, so the pointer must
be checked before using it in savegameRestore().
EMI: Fix crash when talking to Voodoo lady

When asking the Voodoo lady why the island is so empty the engine
quits because the script calls Actor:say_line() with an empty
message.
EMI: Rotate sprite actors to face camera

Sprite actors, e.g. ships of the balloon in the Scumm bar, should
always face the camera, so the rotation part of their matrix has
to be reset before rendering. The code already exists for Grim, but
appears to work for EMI as well.
EMI: Leave sort order unchanged if no sort plane was found

The floor in the Hall of Justice is covered by a solid gray actor
named "trap_plane" which is supposed to be rendered at sort order
100 but gets moved to the foreground by Actor::update(). This patch
avoids changing the actor sort order if no sort plane was found for
the current sector.