Fix freeing monsters not on map

Sometimes we free the monster data, but the monster is not on the
map - usually this happens if the map is full of monsters and a new one
is migrated on the level.

Make m_detach check the monster x coordinate, so it knows not to touch the map
if the monster isn't on it.

src/dog.c

@@ -464,6 +464,7 @@ fail_mon_placement:
             }
             (void) mkcorpstat(CORPSE, (struct monst *) 0, mtmp->data, xlocale,
                               ylocale, CORPSTAT_NONE);
+            mtmp->mx = mtmp->my = -1; /* for mongone, mon is not anywhere */
             mongone(mtmp);
         }
     }

src/mail.c

@@ -418,7 +418,8 @@ struct mail_info *info;
 
 /* zip back to starting location */
 go_back:
-    (void) md_rush(md, start.x, start.y);
+    if (!md_rush(md, start.x, start.y))
+        md->mx = md->my = -1; /* for mongone, md is not on map */
     mongone(md);
 /* deliver some classes of messages even if no daemon ever shows up */
 give_up:

src/mon.c

@@ -1745,6 +1745,8 @@ m_detach(mtmp, mptr)
 struct monst *mtmp;
 struct permonst *mptr; /* reflects mtmp->data _prior_ to mtmp's death */
 {
+    boolean onmap = (mtmp->mx > -1);
+
     if (mtmp == context.polearm.hitmon)
         context.polearm.hitmon = 0;
     if (mtmp->mleashed)
@@ -1753,14 +1755,21 @@ struct permonst *mptr; /* reflects mtmp->data _prior_ to mtmp's death */
     mtmp->mtrapped = 0;
     mtmp->mhp = 0; /* simplify some tests: force mhp to 0 */
     relobj(mtmp, 0, FALSE);
-    remove_monster(mtmp->mx, mtmp->my);
+    if (onmap) {
+        if (mtmp->wormno)
+            remove_worm(mtmp);
+        else
+            remove_monster(mtmp->mx, mtmp->my);
+    }
     if (emits_light(mptr))
         del_light_source(LS_MONSTER, monst_to_any(mtmp));
     if (mtmp->m_ap_type)
         seemimic(mtmp);
-    newsym(mtmp->mx, mtmp->my);
+    if (onmap)
+        newsym(mtmp->mx, mtmp->my);
     unstuck(mtmp);
-    fill_pit(mtmp->mx, mtmp->my);
+    if (onmap)
+        fill_pit(mtmp->mx, mtmp->my);
 
     if (mtmp->isshk)
         shkgone(mtmp);