lib/tag: handle NULL argument to notmuch_tags_valid

Make the behaviour when passed NULL consistent with
notmuch_filenames_valid. The library already passes the result of
notmuch_message_get_tags without checking for NULL, so it should be
handled.

lib/notmuch.h

@@ -2262,6 +2262,9 @@ notmuch_message_properties_destroy (notmuch_message_properties_t *properties);
  * valid string. Whereas when this function returns FALSE,
  * notmuch_tags_get will return NULL.
  *
+ * It is acceptable to pass NULL for 'tags', in which case this
+ * function will always return FALSE.
+
  * See the documentation of notmuch_message_get_tags for example code
  * showing how to iterate over a notmuch_tags_t object.
  */

lib/tags.c

@@ -48,7 +48,7 @@ _notmuch_tags_create (const void *ctx, notmuch_string_list_t *list)
 notmuch_bool_t
 notmuch_tags_valid (notmuch_tags_t *tags)
 {
-    return tags->iterator != NULL;
+    return tags && (tags->iterator != NULL);
 }
 
 const char *

test/T565-lib-tags.sh

@@ -68,7 +68,6 @@ test_expect_equal_file EXPECTED OUTPUT.clean
 restore_database
 
 test_begin_subtest "NULL tags are not valid"
-test_subtest_known_broken
 cat c_head - c_tail <<'EOF' | test_C ${MAIL_DIR}
    {
        notmuch_bool_t valid = TRUE;