accept NULL values in hashtable enumerator

src/libstrongswan/utils/hashtable.c

@@ -331,9 +331,15 @@ static bool enumerate(private_enumerator_t *this, void **key, void **value)
 			pair_t *pair;
 			
 			if (this->current->enumerate(this->current, &pair))
+			{
+				if (key)
 				{
 					*key = pair->key;
+				}
+				if (value)
+				{
 					*value = pair->value;
+				}
 				return TRUE;
 			}
 			this->current->destroy(this->current);