patch 9.0.0340: the 'cmdheight' zero support causes too much trouble

Problem:    The 'cmdheight' zero support causes too much trouble.
Solution:   Revert support for 'cmdheight' being zero.
diff --git a/src/screen.c b/src/screen.c
index eb01fdb..12377d5 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4228,7 +4228,7 @@
     int		nwr_save;
     int		sub_attr;
 
-    do_mode = p_smd && msg_silent == 0 && p_ch > 0
+    do_mode = p_smd && msg_silent == 0
 	    && ((State & MODE_INSERT)
 		|| restart_edit != NUL
 		|| VIsual_active);
@@ -4741,7 +4741,7 @@
     int
 messaging(void)
 {
-    return (!(p_lz && char_avail() && !KeyTyped)) && p_ch > 0;
+    return (!(p_lz && char_avail() && !KeyTyped));
 }
 
 /*