Fix NPC map event

The npc map event should not interfere with the npcs itself. It should
only trigger the dialog with a certain npc.

This fixes a crash at the cavern.txt map at polymorphable at
a514306591951b70eec155938f1216a492145a29
which has this structure:

	[event]
	# Unblock passage
	type=run_once
	npc=gravestone

	[npc]
	type=gravestone

This lead to a crash as the event referencing the npc, was loaded prior to
any npc loaded.
Merge pull request #598 from stefanbeller/npc

Fix NPC map event