[balsa] send: Fix a compiler warning

[Peter Bloomfield <peterb src gnome org>]

commit 7b93e90c639267dd14aed2efc2428314f918e632
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.

 ChangeLog       | 11 +++++++++++
 libbalsa/send.c |  6 ++++--
 2 files changed, 15 insertions(+), 2 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 653d59768..29dcf8ead 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2019-04-20  Peter Bloomfield  <pbloomfield bellsouth net>
+
+       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.
+
 2019-04-18  Albrecht Dreß  <albrecht dress arcor de>
 
        Fix the build after commit e92ec3140a1c81dbf86d90d0155cb2e08f519db2
diff --git a/libbalsa/send.c b/libbalsa/send.c
index a68f1b079..e42a7c840 100644
--- a/libbalsa/send.c
+++ b/libbalsa/send.c
@@ -1602,9 +1602,11 @@ 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... */