fix #K3627 - impossible placing long worm at <0,0>

When migrating, a long worm is removed from the map to take off the
tail, then its head is put back to be treated like other monsters.
If that occurred when being forced to re-migrate during failure to
arrive from a prior migration, it wouldn't have valid coordinates
and the place_monster attempt produced an impossible warning.
(Other types of monsters don't get removed and put back so didn't
trigger the problem.)

The routine to format a monster when the data is suspect mistakenly
thought it was dealing with a long worm tail because the monster
didn't match level.monsters[0][0], so the warning inaccurately
reported the problem as "placing long worm tail".

doc/fixes3-7-0.txt

@@ -937,6 +937,10 @@ the u.ustuck hierarchy is:  swallowed by ustuck, hero poly'd into sticky form
 when a vampire gains levels and grows into a vampire lord, change its cham
 	field (shapechanger base type) from vampire to vampire lord so that if
 	it revives in base form it won't revert to plain vampire
+a migrating long worm that couldn't arrive could be placed at <0,0> while
+	setting up another migration attempt to the level, triggering
+        impossible "trying to place long worm tail at <0,0> mstate:8 on level"
+	(message is confused; it should say "long worm" without tail)
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/do_name.c

@@ -2186,7 +2186,7 @@ minimal_monnam(struct monst *mon, boolean ckloc)
         Sprintf(outbuf, "[Invalid mon->data %s >= %s]",
                 fmt_ptr((genericptr_t) mon->data),
                 fmt_ptr((genericptr_t) &mons[NUMMONS]));
-    } else if (ckloc && ptr == &mons[PM_LONG_WORM]
+    } else if (ckloc && ptr == &mons[PM_LONG_WORM] && mon->mx
                && g.level.monsters[mon->mx][mon->my] != mon) {
         Sprintf(outbuf, "%s <%d,%d>",
                 pmname(&mons[PM_LONG_WORM_TAIL], Mgender(mon)),

src/dog.c

@@ -609,8 +609,10 @@ mon_leave(struct monst *mtmp)
            more segments than can fit in that field gets truncated */
         num_segs = min(cnt, MAX_NUM_WORMS - 1);
         wormgone(mtmp);
-        /* put the head back */
-        place_monster(mtmp, mx, my);
+        /* put the head back; note: mtmp might not be on the map if this
+           is happening during a failed attempt to migrate to this level */
+        if (mx)
+            place_monster(mtmp, mx, my);
     }
 
     return num_segs;

src/mon.c

@@ -3293,6 +3293,12 @@ m_into_limbo(struct monst *mtmp)
 static void
 migrate_mon(struct monst *mtmp, xchar target_lev, xchar xyloc)
 {
+    if (!mtmp->mx) {
+        /* this was a failed arrival attempt from a prior migration;
+           force mtmp to temporarily have a valid location when starting
+           its new migration */
+        mtmp->mx = u.ux, mtmp->my = u.uy;
+    }
     unstuck(mtmp);
     mdrop_special_objs(mtmp);
     migrate_to_level(mtmp, target_lev, xyloc, (coord *) 0);