Use-after-free with engulfer in xkilled #938

If you were on a level teleporter, the spoteffects() call after
the hero gets expelled could end up going to a new level and
freeing all the monst chains from the level you were originally
engulfed on.

    #0 0xba0507 in free
    #1 0x87feda in dealloc_monst src/mon.c:2369
    #2 0x880a02 in dmonsfree src/mon.c:2194
    #3 0x9a7aa2 in savelev_core src/save.c:507
    #4 0x9a7a21 in savelev src/save.c:466
    #5 0x71eb9d in goto_level src/do.c:1483
    #6 0x71833f in deferred_goto src/do.c:1903
    #7 0xa2533f in level_tele src/teleport.c:1117
    #8 0xa2567b in level_tele_trap src/teleport.c:1198
    #9 0xa5c007 in trapeffect_level_telep src/trap.c:1861
    #10 0xa5f856 in trapeffect_selector src/trap.c:2497
    #11 0xa47497 in dotrap src/trap.c:2586
    #12 0x7d669b in spoteffects src/hack.c:2859
    #13 0x89d495 in xkilled src/mon.c:3187

The latter parts of xkilled() after the spoteffects() call would
then attempt to dereference the free'd monst pointer.

Save a copy of the monst struct prior to spoteffects() if you were
expelled, then point at the reference copy afterwards.

Resolves #938

src/mon.c

@@ -3044,6 +3044,7 @@ xkilled(
 {
     int tmp, mndx;
     coordxy x = mtmp->mx, y = mtmp->my;
+    struct monst museum = cg.zeromonst;
     struct permonst *mdat;
     struct obj *otmp;
     struct trap *t;
@@ -3183,8 +3184,16 @@ xkilled(
             }
         }
     }
-    if (wasinside)
+
+    if (wasinside) {
+        /* spoteffects() can end up clearing the level of monsters; grab a copy */
+        museum = *mtmp;
+        museum.nmon = 0;
+        museum.minvent = 0;
+        museum.mextra = 0;
         spoteffects(TRUE); /* poor man's expels() */
+        mtmp = &museum; /* use the reference copy now */
+    }
     /* monster is gone, corpse or other object might now be visible */
     newsym(x, y);