[glib] gdatetime: Fix a potential overflow in overflow calculations

[Philip Withnall <pwithnall src gnome org>]

commit 2db7aa479919dc7ae8742f52ad5d54191a7282fd
Author: Philip Withnall <withnall endlessm com>
Date:   Fri Jun 16 12:31:58 2017 +0100

    gdatetime: Fix a potential overflow in overflow calculations
    
    I can’t remember whether glong (tv.tv_sec) needs to be explicitly
    promoted to gint64 here, or whether C does it automatically. Safer to
    make the cast explicit to avoid overflow issues on 32-bit platforms,
    where glong is 32-bit.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=783841

 glib/gdatetime.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/glib/gdatetime.c b/glib/gdatetime.c
index 7695adf..745a32a 100644
--- a/glib/gdatetime.c
+++ b/glib/gdatetime.c
@@ -128,7 +128,7 @@ struct _GDateTime
 #define INSTANT_TO_UNIX(instant) \
   ((instant)/USEC_PER_SECOND - UNIX_EPOCH_START * SEC_PER_DAY)
 #define UNIX_TO_INSTANT(unix) \
-  (((unix) + UNIX_EPOCH_START * SEC_PER_DAY) * USEC_PER_SECOND)
+  (((gint64) (unix) + UNIX_EPOCH_START * SEC_PER_DAY) * USEC_PER_SECOND)
 #define UNIX_TO_INSTANT_IS_VALID(unix) \
   ((gint64) (unix) <= INSTANT_TO_UNIX (G_MAXINT64))