Mail: Fix crash when closing window before IMAP::Client connects

`MailWidget::m_imap_client` is only assigned after a connection is
established, so the user might close the main window before that
happens, especially if the connection hangs for whatever reason.
Now we check the `OwnPtr` before working with it.

Author: vkoskiv

Userland/Applications/Mail/MailWidget.cpp

@@ -163,6 +163,10 @@ bool MailWidget::connect_and_login()
 
 void MailWidget::on_window_close()
 {
+    if (!m_imap_client) {
+        // User closed main window before a connection was established
+        return;
+    }
     auto response = move(MUST(m_imap_client->send_simple_command(IMAP::CommandType::Logout)->await()).release_value().get<IMAP::SolidResponse>());
     VERIFY(response.status() == IMAP::ResponseStatus::OK);