g_cond_timed_wait: support NULL time parameter

It was undocumented, but this used to mean "wait forever".  Looks like
we have some uses of this internally and there may be others in the
wild...

glib/deprecated/gthread-deprecated.c

@@ -1551,6 +1551,12 @@ g_cond_timed_wait (GCond    *cond,
 {
   gint64 end_time;
 
+  if (abs_time == NULL)
+    {
+      g_cond_wait (cond, mutex);
+      return TRUE;
+    }
+
   end_time = abs_time->tv_sec;
   end_time *= 1000000;
   end_time += abs_time->tv_usec;