[balsa/wip/gtk4] send: Fix a compiler warning

[Peter Bloomfield <peterb src gnome org>]

commit 3c6d41299c5d86416f91f2ef414b84b733a315f5
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Sat Apr 20 21:46:20 2019 -0400

    send: Fix a compiler warning
    
    * libbalsa/send.c (libbalsa_set_message_id): strncpy() with n ==
      sizeof(dest) generates:
      "error: ‘__builtin_strncpy’ specified bound 16 equals
      destination size [-Werror=stringop-truncation]"
      We now specify one fewer bytes, and explicitly nullify the last
      byte.

 libbalsa/send.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/libbalsa/send.c b/libbalsa/send.c
index 0f8181277..8f081c277 100644
--- a/libbalsa/send.c
+++ b/libbalsa/send.c
@@ -1589,9 +1589,11 @@ libbalsa_set_message_id(GMimeMessage *mime_message)
         /* initialise some stuff on first-time use... */
         rand = g_rand_new_with_seed((guint32) time(NULL));
         strncpy(id_data.user_name, g_get_user_name(),
-                sizeof(id_data.user_name));
+                sizeof(id_data.user_name) - 1);
+        id_data.user_name[sizeof(id_data.user_name) - 1] = '\0';
         strncpy(id_data.host_name, g_get_host_name(),
-                sizeof(id_data.host_name));
+                sizeof(id_data.host_name) - 1);
+        id_data.host_name[sizeof(id_data.host_name) - 1] = '\0';
     }
 
     /* get some randomness... */