[epiphany] ephy-shell: Fix compilation warning

[Jan-Michael Brummer <jbrummer src gnome org>]

commit 3865933c86d9759581dabeb8e7bedd537c55226b
Author: Jan-Michael Brummer <jan brummer tabos org>
Date:   Sun Jun 14 12:06:42 2020 +0200

    ephy-shell: Fix compilation warning
    
    1481 ../src/ephy-shell.c: In function ‘webkit_notification_closed_cb’:
    1482 /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘id’ may be used uninitialized in this 
function [-Werror=maybe-uninitialized]
    1483    28 |   g_free (*pp);
    1484       |   ^~~~~~~~~~~~
    1485 ../src/ephy-shell.c:913:20: note: ‘id’ was declared here
    1486   913 |   g_autofree char *id;

 src/ephy-shell.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/ephy-shell.c b/src/ephy-shell.c
index 4aae6510a..3c5777b4b 100644
--- a/src/ephy-shell.c
+++ b/src/ephy-shell.c
@@ -910,7 +910,7 @@ webkit_notification_closed_cb (WebKitNotification *notification,
                                gpointer            user_data)
 {
   EphyShell *shell = ephy_shell_get_default ();
-  g_autofree char *id;
+  g_autofree char *id = NULL;
 
   id = g_strdup_printf ("%" G_GUINT64_FORMAT, webkit_notification_get_id (notification));