Make Juiblex resist magical digging from inside

Juiblex should be a little bit harder to beat than zapping a digging
wand once you've gotten swallowed and then whacking him once.

Make his HP halve every time you zap digging instead of setting it
to 1.

Change via UnNetHack.

src/dig.c

@@ -1409,7 +1409,10 @@ zap_dig()
             if (is_animal(mtmp->data))
                 You("pierce %s %s wall!", s_suffix(mon_nam(mtmp)),
                     mbodypart(mtmp, STOMACH));
-            mtmp->mhp = 1; /* almost dead */
+            if (unique_corpstat(mtmp->data))
+                mtmp->mhp = (mtmp->mhp + 1) / 2;
+            else
+                mtmp->mhp = 1; /* almost dead */
             expels(mtmp, mtmp->data, !is_animal(mtmp->data));
         }
         return;