Fix couple cases of unhiding monsters

Hidden monster might be forced to move to a location where it
can't hide, perhaps because it's mostly surrounded by other monsters.

Hidden monster in a pit under items, getting hit by a rolling boulder,
the boulder will fill the pit burying the items, making the monster
unable to hide there.

src/dig.c

@@ -1908,6 +1908,7 @@ bury_objs(int x, int y)
     /* don't expect any engravings here, but just in case */
     del_engr_at(x, y);
     newsym(x, y);
+    maybe_unhide_at(x, y);
 
     if (costly && loss) {
         You("owe %s %ld %s for burying merchandise.", shkname(shkp), loss,