Prevent phaseable monster hiding deep in undiggable walls

The outer edge of a random dungeon level can have undiggable
walls.  Phaseable monsters, such as earth elementals, could
hide deep inside that boundary.  Turn the walls beyond the
first layer of non-diggable walls also non-phaseable.

doc/fixes3-7-0.txt

@@ -1592,6 +1592,7 @@ one orc-town shaman has a higher level, affecting spellcasting
 hero has a small chance of catching items thrown at them
 wizard mode: history menu for #wizwish and WIZKIT
 monster priests and wizards did not cast spells
+prevent phaseable monsters hiding deep inside nondiggable walls
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/mkmaze.c

@@ -1426,8 +1426,9 @@ get_level_extends(
     *bottom = ymax;
 }
 
-/* put a non-diggable boundary around the initial portion of a level map.
+/* put a non-diggable/non-phaseable boundary around the initial portion
- * assumes that no level will initially put things beyond the isok() range.
+ * of a level map. assumes that no level will initially put things
+ * beyond the isok() range.
  *
  * we can't bound unconditionally on the last line with something in it,
  * because that something might be a niche which was already reachable,
@@ -1449,9 +1450,14 @@ bound_digging(void)
 
     for (x = 0; x < COLNO; x++)
         for (y = 0; y < ROWNO; y++)
-            if (IS_STWALL(levl[x][y].typ)
+            if (IS_STWALL(levl[x][y].typ)) {
-                && (y <= ymin || y >= ymax || x <= xmin || x >= xmax))
+                /* undiggable walls at edges, ... */
-                levl[x][y].wall_info |= W_NONDIGGABLE;
+                if (y <= ymin || y >= ymax || x <= xmin || x >= xmax)
+                    levl[x][y].wall_info |= W_NONDIGGABLE;
+                /* one tile past that, everything is also unphaseable */
+                if (y < ymin || y > ymax || x < xmin || x > xmax)
+                    levl[x][y].wall_info |= W_NONPASSWALL;
+            }
 }
 
 void