KYRA: Extend safety check inside AdLibDriver::getProgram.

This also removes an TODO and replaces it with a comment explaining this
safety check.
KYRA: Add safety checks for getProgram/getInstrument return values.

This fixes a crash in Hand of Fate when scaring off the rat by using the
swampsnake potion on Zanthia. This crash is timing sensitive since the game is
stopping the offending sound resource at this point. If it still gets to the
instruction setting up an invalid instrument, it will crash ScummVM with in
invalid read.
KYRA: Constify base type for some pointers in AdLibDriver.
KYRA: Slight cleanup.