Fix crash in DES_DECRYPT(NULL). (Bug #12757)

sql/item_strfunc.cc:
  Don't call res->length() when res may be NULL because args[0] is a NULL.

sql/item_strfunc.cc

@@ -472,11 +472,11 @@ String *Item_func_des_decrypt::val_str(String *str)
   struct st_des_keyblock keyblock;
   struct st_des_keyschedule keyschedule;
   String *res= args[0]->val_str(str);
-  uint length=res->length(),tail;
+  uint length,tail;
 
-  if ((null_value=args[0]->null_value))
+  if ((null_value= args[0]->null_value))
     return 0;
-  length=res->length();
+  length= res->length();
   if (length < 9 || (length % 8) != 1 || !((*res)[0] & 128))
     return res;				// Skip decryption if not encrypted