[balsa] mailbox_imap: Avoid a sign-compare warning

[Peter Bloomfield <peterb src gnome org>]

commit 61051ef0ef364eb7ed1b2c376177e42bd59091ae
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Tue Mar 15 11:49:56 2022 -0400

    mailbox_imap: Avoid a sign-compare warning
    
    Reported by Andreas and suggested fix from Albrecht:
    https://mail.gnome.org/archives/balsa-list/2022-March/msg00006.html

 libbalsa/mailbox_imap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/libbalsa/mailbox_imap.c b/libbalsa/mailbox_imap.c
index 3e98a1b87..929e3a2ef 100644
--- a/libbalsa/mailbox_imap.c
+++ b/libbalsa/mailbox_imap.c
@@ -2325,9 +2325,9 @@ libbalsa_mailbox_imap_fetch_headers(LibBalsaMailbox *mailbox,
 {
     LibBalsaMailboxImap *mimap = LIBBALSA_MAILBOX_IMAP(mailbox);
     ImapResponse rc;
-    glong msgno;
+    guint msgno;
 
-    msgno = libbalsa_message_get_msgno(message);
+    msgno = (guint) libbalsa_message_get_msgno(message);
     /* If message numbers are out of sync with the mail store,
      * just skip the message: */
     if (msgno > imap_mbox_handle_get_exists(mimap->handle))