EMI: Ignore Actor:walkTo() if destination is unreachable

The path to the Curch of LeChuck has bogus coordinates as out point, so
the game will walk Guybrush to an invalid position when entering that
set. This patch will detect when no path can be found to the destination
position and ignore the walkTo() request in that case.

The behavior for Grim is unchanged in order to avoid the risk of breaking
the game. If the warning is never triggered the distinction can be removed.
Merge pull request #794 from inguin/emi-walkto

GRIM/EMI: Ignore Actor:walkTo() if destination is unreachable