patch 9.0.0114: the command line takes up space even when not used

Problem:    The command line takes up space even when not used.
Solution:   Allow for 'cmdheight' to be set to zero. (Shougo Matsushita,
            closes #10675, closes #940)
diff --git a/src/screen.c b/src/screen.c
index 1093b7c..1f223f9 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4726,7 +4726,7 @@
     int
 messaging(void)
 {
-    return (!(p_lz && char_avail() && !KeyTyped));
+    return (!(p_lz && char_avail() && !KeyTyped)) && p_ch > 0;
 }
 
 /*