Fix missing deref in C14N (#14203)

Follow-up for 30a0b03, which didn't fix
all places. This is the last remaining place.

Author: nielsdos

ext/dom/node.c

@@ -1667,6 +1667,7 @@ static void dom_canonicalization(INTERNAL_FUNCTION_PARAMETERS, int mode) /* {{{
 			inclusive_ns_prefixes = safe_emalloc(zend_hash_num_elements(Z_ARRVAL_P(ns_prefixes)) + 1,
 				sizeof(xmlChar *), 0);
 			ZEND_HASH_FOREACH_VAL(Z_ARRVAL_P(ns_prefixes), tmpns) {
+				ZVAL_DEREF(tmpns);
 				if (Z_TYPE_P(tmpns) == IS_STRING) {
 					inclusive_ns_prefixes[nscount++] = (xmlChar *) Z_STRVAL_P(tmpns);
 				}

ext/dom/tests/DOMNode_C14N_references.phpt

@@ -38,4 +38,4 @@ unset($v);
 echo $doc->C14N(true, false, $xpath, $prefixes);
 ?>
 --EXPECT--
-<contain xmlns="http://www.example.com/ns/foo"><bar></bar><bar></bar></contain>
+<contain xmlns="http://www.example.com/ns/foo" xmlns:test="urn::test"><bar></bar><bar></bar></contain>