Fix: vampire shapeshifting in a door

Someone mentioned in #nethack today that a vampire had turned into a fog
cloud, moved under a door, then turned back into a vampire while still
sharing a space with a closed door.  There already existed some code
intended to prevent this in cases where the vampire is killed in one
form and revives in another, but voluntary transformations (from
decide_to_shapeshift) weren't included.  The existing code in mondead
and vamp_stone also apparently missed a minor edge case: because the
code between the definition of in_door and its use included an expels
call, it would be outdated/incorrect in cases where expels() placed the
fog cloud onto a closed door.

src/mon.c

@@ -2494,7 +2494,7 @@ lifesaved_monster(struct monst* mtmp)
 DISABLE_WARNING_FORMAT_NONLITERAL
 
 void
-mondead(register struct monst* mtmp)
+mondead(struct monst *mtmp)
 {
     struct permonst *mptr;
     boolean be_sad;
@@ -2516,13 +2516,11 @@ mondead(register struct monst* mtmp)
             && !(g.mvitals[mndx].mvflags & G_GENOD)) {
             coord new_xy;
             char buf[BUFSZ];
-            boolean in_door = (amorphous(mtmp->data)
-                               && closed_door(mtmp->mx, mtmp->my)),
             /* alternate message phrasing for some monster types */
-                spec_mon = (nonliving(mtmp->data)
+            boolean spec_mon = (nonliving(mtmp->data)
                                 || noncorporeal(mtmp->data)
                                 || amorphous(mtmp->data)),
-                spec_death = (g.disintegested /* disintegrated or digested */
+                    spec_death = (g.disintegested /* disintegrated/digested */
                                   || noncorporeal(mtmp->data)
                                   || amorphous(mtmp->data));
             int x = mtmp->mx, y = mtmp->my;
@@ -2546,7 +2544,9 @@ mondead(register struct monst* mtmp)
                 else
                     uunstick();
             }
-            if (in_door) {
+            /* if fog cloud is on a closed door space, move it to a more
+               appropriate spot for its intended new form */
+            if (amorphous(mtmp->data) && closed_door(mtmp->mx, mtmp->my)) {
                 if (enexto(&new_xy, mtmp->mx, mtmp->my, &mons[mndx]))
                     rloc_to(mtmp, new_xy.x, new_xy.y);
             }
@@ -3230,8 +3230,6 @@ vamp_stone(struct monst* mtmp)
         if (mndx >= LOW_PM && mndx != monsndx(mtmp->data)
             && !(g.mvitals[mndx].mvflags & G_GENOD)) {
             char buf[BUFSZ];
-            boolean in_door = (amorphous(mtmp->data)
-                               && closed_door(mtmp->mx, mtmp->my));
 
             /* construct a format string before transformation */
             Sprintf(buf, "The lapidifying %s %s %s",
@@ -3249,7 +3247,7 @@ vamp_stone(struct monst* mtmp)
             /* this can happen if previously a fog cloud */
             if (engulfing_u(mtmp))
                 expels(mtmp, mtmp->data, FALSE);
-            if (in_door) {
+            if (amorphous(mtmp->data) && closed_door(mtmp->mx, mtmp->my)) {
                 coord new_xy;
 
                 if (enexto(&new_xy, mtmp->mx, mtmp->my, &mons[mndx])) {
@@ -4123,6 +4121,14 @@ decide_to_shapeshift(struct monst* mon, int shiftflags)
                     dochng = (ptr != mon->data);
                 }
             }
+            if (dochng && amorphous(mon->data)
+                && closed_door(mon->mx, mon->my)) {
+                coord new_xy;
+
+                if (enexto(&new_xy, mon->mx, mon->my, ptr)) {
+                    rloc_to(mon, new_xy.x, new_xy.y);
+                }
+            }
         } else {
             if (mon->mhp >= 9 * mon->mhpmax / 10 && !rn2(6)
                 && (!canseemon(mon)