gmain: remove unnecessary initialization of *timeout in prepare() callbacks

Note that the prepare callback only has one caller, which pre-initializes
the timeout argument to -1. That may be an implementation detail and not
publicly promised, but it wouldn't make sense to do it any other way in
the caller.

Also, note that g_unix_signal_watch_prepare() and the UNIX branch of
g_child_watch_prepare() already relied on that.

gio/gsocket.c

@@ -4003,8 +4003,6 @@ socket_source_prepare (GSource *source,
 {
   GSocketSource *socket_source = (GSocketSource *)source;
 
-  *timeout = -1;
-
 #ifdef G_OS_WIN32
   if ((socket_source->pollfd.revents & G_IO_NVAL) != 0)
     return TRUE;

glib/giounix.c

@@ -135,8 +135,6 @@ g_io_unix_prepare (GSource  *source,
   GIOUnixWatch *watch = (GIOUnixWatch *)source;
   GIOCondition buffer_condition = g_io_channel_get_buffer_condition (watch->channel);
 
-  *timeout = -1;
-
   /* Only return TRUE here if _all_ bits in watch->condition will be set
    */
   return ((watch->condition & buffer_condition) == watch->condition);

glib/giowin32.c

@@ -709,8 +709,6 @@ g_io_win32_prepare (GSource *source,
   GIOWin32Channel *channel = (GIOWin32Channel *)watch->channel;
   int event_mask;
   
-  *timeout = -1;
-  
   if (channel->debug)
     g_print ("g_io_win32_prepare: source=%p channel=%p", source, channel);
 

glib/gmain.c

@@ -5510,7 +5510,6 @@ g_child_watch_prepare (GSource *source,
 		       gint    *timeout)
 {
 #ifdef G_OS_WIN32
-  *timeout = -1;
   return FALSE;
 #else /* G_OS_WIN32 */
   {

glib/tests/mainloop.c

@@ -37,6 +37,8 @@ cb (gpointer data)
 static gboolean
 prepare (GSource *source, gint *time)
 {
+  g_assert_nonnull (time);
+  g_assert_cmpint (*time, ==, -1);
   return FALSE;
 }
 static gboolean