avoid impossible() with inbound migrating mon

include/monst.h

@@ -64,6 +64,7 @@ enum m_ap_types {
 #define MON_ENDGAME_FREE 0x20
 #define MON_ENDGAME_MIGR 0x40
 #define MON_OBLITERATE   0x80
+#define MON_STILL_ARRIVING 0x100
 
 #define M_AP_TYPMASK  0x7
 #define M_AP_F_DKNOWN 0x8

src/display.c

@@ -1505,6 +1505,8 @@ see_monsters(void)
     for (mon = fmon; mon; mon = mon->nmon) {
         if (DEADMONSTER(mon))
             continue;
+        if ((mon->mstate & MON_STILL_ARRIVING) != 0)
+            continue;
         newsym(mon->mx, mon->my);
         if (mon->wormno)
             see_wsegs(mon);

src/dog.c

@@ -427,6 +427,7 @@ mon_arrive(struct monst *mtmp, int when)
     stairway *stway;
     d_level fromdlev;
 
+    mtmp->mstate |= MON_STILL_ARRIVING;
     mtmp->nmon = fmon;
     fmon = mtmp;
     if (mtmp->isshk)
@@ -475,6 +476,7 @@ mon_arrive(struct monst *mtmp, int when)
             rloc_to(mtmp, u.ux, u.uy);
         else
             mnexto(mtmp, RLOC_NOMSG);
+        mtmp->mstate &= ~MON_STILL_ARRIVING;
         return;
     } else if (when == Wiz_arrive) {
         /* resurrect() is bringing existing wizard to harass the hero */
@@ -604,6 +606,8 @@ mon_arrive(struct monst *mtmp, int when)
 
     mtmp->mx = 0; /*(already is 0)*/
     mtmp->my = xyflags;
+    mtmp->mstate &= ~MON_STILL_ARRIVING;
+
     if (xlocale)
         failed_to_place = !mnearto(mtmp, xlocale, ylocale, FALSE, RLOC_NOMSG);
     else