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: Fix save/restore when compiled without MP3 support

Saving and restoring the emisound state fails when residualvm
is compiled without libmad MP3 support. This patch makes
openSound() return successfully if the stream is valid but MP3
support is not available.
EMI: Fix fullscreen toggle in main menu

When the main menu is initially opened it will call ImPushState() before
any music is playing. This results in a NULL entry on EMISound's state
stack. saveState/restoreState must be able to deal with this situation
because they are used when toggling fullscreen.
Merge pull request #759 from inguin/emi-fixes

EMI: Various small bugfixes