gdatetime: Track timezone length as an unsigned size_t

It’s guaranteed to be in (0, length] by the calculations above.

This avoids the possibility of integer overflow through `gssize` not
being as big as `size_t`.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/gdatetime.c

@@ -1398,7 +1398,8 @@ parse_iso8601_date (const gchar *text, gsize length,
 static GTimeZone *
 parse_iso8601_timezone (const gchar *text, gsize length, size_t *tz_offset)
 {
-  gint i, tz_length, offset_hours, offset_minutes;
+  size_t tz_length;
+  gint i, offset_hours, offset_minutes;
   gint offset_sign = 1;
   GTimeZone *tz;