Fix purple worm ending up in solid wall

If you're in a vault, the vault guard enters, and your tame purple worm
swallows and kills the guard, the worm ended up inside the vault wall.
Instead return the attacker to the old location, if the defender's
position isn't good.

doc/fixes37.0

@@ -94,6 +94,7 @@ unicorn corpses and wraith corpses could be sacrificed even if "too old"
 hero polymorphed into a hider and hiding was not unhidden when teleporting
 impose tighter restraints on 'summon nasties', both for spellcasting monsters
 	and post-Wizard harassment
+prevent swallowing monster ending up in a solid wall if it killed vault guard
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/mhitm.c

@@ -765,6 +765,8 @@ register struct attack *mattk;
          *  magr from level.monsters[mdef->mx][mdef->my].  We need to
          *  put it back and display it.  -kd
          */
+        if (!goodpos(dx, dy, magr, MM_IGNOREWATER))
+            dx = ax, dy = ay;
         place_monster(magr, dx, dy);
         newsym(dx, dy);
         /* aggressor moves to <dx,dy> and might encounter trouble there */