notmuch-show: detect NULL pointer returned from notmuch_query_search_threads

We want to return an error status, not 0 or (worse) segfault.

notmuch-show.c

@@ -1015,9 +1015,13 @@ do_show (void *ctx,
     notmuch_messages_t *messages;
     notmuch_status_t status, res = NOTMUCH_STATUS_SUCCESS;
 
+    threads = notmuch_query_search_threads (query);
+    if (! threads)
+	return 1;
+
     sp->begin_list (sp);
 
-    for (threads = notmuch_query_search_threads (query);
+    for ( ;
 	 notmuch_threads_valid (threads);
 	 notmuch_threads_move_to_next (threads))
     {

test/T520-show.sh

@@ -6,7 +6,6 @@ test_description='"notmuch show"'
 add_email_corpus
 
 test_begin_subtest "exit code for show invalid query"
-test_subtest_known_broken
 notmuch show foo..
 exit_code=$?
 test_expect_equal 1 $exit_code