patch 8.1.0840: getchar(0) never returns a character in the terminal

Problem:    getchar(0) never returns a character in the terminal.
Solution:   Call wait_func() at least once.
diff --git a/src/gui_gtk_x11.c b/src/gui_gtk_x11.c
index 6882733..e1de83a 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -6317,10 +6317,11 @@
 
     timed_out = FALSE;
 
-    /* this timeout makes sure that we will return if no characters arrived in
-     * time */
-    if (wtime > 0)
-	timer = timeout_add(wtime, input_timer_cb, &timed_out);
+    // This timeout makes sure that we will return if no characters arrived in
+    // time. If "wtime" is zero just use one.
+    if (wtime >= 0)
+	timer = timeout_add(wtime <= 0 ? 1L : wtime,
+						   input_timer_cb, &timed_out);
     else
 	timer = 0;