Fix strange object mimic in shop sanity error

Shop population code set the mimic shape to strange object
without checking for protection from shape changers.
Let set_mimic_sym (via makemon) handle it correctly instead.

src/makemon.c

@@ -2212,6 +2212,10 @@ set_mimic_sym(register struct monst *mtmp)
      */
 
     } else if (rt >= SHOPBASE) {
+        if (rn2(10) >= depth(&u.uz)) {
+            s_sym = S_MIMIC_DEF; /* -> STRANGE_OBJECT */
+            goto assign_sym;
+        }
         s_sym = get_shop_item(rt - SHOPBASE);
         if (s_sym < 0) {
             ap_type = M_AP_OBJECT;