lib: add talloc reference from string map iterator to map

This is needed so that when the map is modified during traversal, and
thus unlinked by the database code, the map is not disposed of until the
iterator is done with it.

lib/string-map.c

@@ -170,6 +170,9 @@ _notmuch_string_map_iterator_create (notmuch_string_map_t *map, const char *key,
     if (unlikely (iter == NULL))
 	return NULL;
 
+    if (unlikely (talloc_reference (iter, map) == NULL))
+	return NULL;
+
     iter->key = talloc_strdup (iter, key);
     iter->exact = exact;
     iter->current = bsearch_first (map->pairs, map->length, key, exact);