fix github issue #1192 - engulfers vs iron bars

Issue reported by Umbire:  an engulfing monster capable of passing
through iron bars (vortices and air elemental) could do so while
carrying the hero.

Prevent an engulfer from doing that unless the hero happens to be
polymorphed into a subset of the types of monsters that can move to
iron bar locations.

Fixes #1192

doc/fixes3-7-0.txt

@@ -1346,6 +1346,8 @@ if polymorphing a potion turned it into oil, dipping potions produced oil
 	set of the original potion's age (turn it was created) and could burn
 	for an arbitrarily long time if applied; conversely, polymophing oil
 	into non-oil kept its relative age (no noticeable effect though)
+an engulfer capable of passing through iron bars could do so even when hero
+	was engulfed
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/mon.c

@@ -1873,7 +1873,11 @@ mon_allowflags(struct monst* mtmp)
         allowflags |= OPENDOOR;
     if (can_unlock)
         allowflags |= UNLOCKDOOR;
-    if (passes_bars(mtmp->data))
+    if (passes_bars(mtmp->data)
+        /* restrict engulfer or holder who might try to pass iron bars while
+           carrying hero; accept small subset for poly'd hero passes_bars() */
+        && (mtmp != u.ustuck || (unsolid(gy.youmonst.data)
+                                 || verysmall(gy.youmonst.data))))
         allowflags |= ALLOW_BARS;
 #if 0   /* can't do this here; leave it for mfndpos() */
     if (is_displacer(mtmp->data))