[nautilus] Apply suggestion to src/nautilus-directory-async.c

[Carlos Soriano <csoriano src gnome org>]

commit 2ea9fee81a4461c297534d012b08fac7d58f69fa
Author: Carlos Soriano <csoriano1618+gnome gmail com>
Date:   Fri Feb 22 11:58:45 2019 +0000

    Apply suggestion to src/nautilus-directory-async.c

 src/nautilus-directory-async.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/nautilus-directory-async.c b/src/nautilus-directory-async.c
index d4124d469..9f3033157 100644
--- a/src/nautilus-directory-async.c
+++ b/src/nautilus-directory-async.c
@@ -2037,7 +2037,7 @@ lookup_monitors (GHashTable   *monitor_table,
                  NautilusFile *file)
 {
     /* To find monitors monitoring all files, use lookup_all_files_monitors. */
-    g_assert (file);
+    g_return_val_if_fail (file, NULL);
 
     return g_hash_table_lookup (monitor_table, file);
 }