gdatetime: Silence -Wmaybe-uninitialized

GCC 6.3.1 thinks that tmp is being used uninitialized:
  gdatetime.c: In function ‘format_ampm’:
  gdatetime.c:2248:7: warning: ‘tmp’ may be used uninitialized in this
    function [-Wmaybe-uninitialized]
         g_free (tmp);
         ^~~~~~~~~~~~

It is not an actual problem because the code in question is guarded by
"if (!locale_is_utf8)" and "#if defined (HAVE_LANGINFO_TIME)", and it
does get initialized under those circumstances. Still, it is a small
price to pay for a cleaner build and having actual problems stand out
more prominently.

https://bugzilla.gnome.org/show_bug.cgi?id=785438

glib/gdatetime.c

@@ -2220,7 +2220,7 @@ format_ampm (GDateTime *datetime,
              gboolean   uppercase)
 {
   const gchar *ampm;
-  gchar       *tmp, *ampm_dup;
+  gchar       *tmp = NULL, *ampm_dup;
   gsize        len;
 
   ampm = GET_AMPM (datetime);