Remember position of monster you hit with polearm

If you hit a monster with a polearm, following polearm applications
will try to automatically suggest the monster's position

src/save.c

@@ -1124,8 +1124,13 @@ register struct monst *mtmp;
 	    }
 	    if (mtmp->minvent)
 		saveobjchn(fd,mtmp->minvent,mode);
-	    if (release_data(mode))
+	    if (release_data(mode)) {
+		if (mtmp == context.polearm.hitmon) {
+		    context.polearm.m_id = mtmp->m_id;
+		    context.polearm.hitmon = NULL;
+		}
 		dealloc_monst(mtmp);
+	    }
 	    mtmp = mtmp2;
 	}
 	if (perform_bwrite(mode))