Clear all room data

Restoring a level cleared residence pointer from those subrooms
which were in the restored level, but if the previous level had
more subrooms, those pointers weren't cleared.
This caused weird problems when a shopkeeper data was looked
up based on the subroom number.

Just to be safe, clear all the room data when freeing the level.

src/save.c

@@ -523,6 +523,9 @@ savelev(NHFILE* nhfp, xchar lev)
         g.level.buriedobjlist = 0;
         g.billobjs = 0;
         stairway_free_all();
+        (void) memset(g.rooms, 0, sizeof(g.rooms));
+        g.nroom = g.nsubroom = 0;
+        g.rooms[0].hx = g.subrooms[0].hx = -1;
         /* level.bonesinfo = 0; -- handled by savecemetery() */
     }
     save_engravings(nhfp);