Fix for xml_easy_content() for nodes with no content.

Instead of crushing xml_easy_content() returns now NULL when there's no content.

src/xml.c

@@ -143,6 +143,9 @@ static _Bool xml_string_equals(struct xml_string* a, struct xml_string* b) {
  * [PRIVATE]
  */
 static uint8_t* xml_string_clone(struct xml_string* s) {
+	if (!s)
+		return 0;
+
 	uint8_t* clone = calloc(s->length + 1, sizeof(uint8_t));
 
 	xml_string_copy(s, clone, s->length);