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/normal.c b/src/normal.c
index 932d723..4033d7d 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -1798,6 +1798,9 @@
 {
     int	    len;
 
+    if (p_ch == 0)
+	return;
+
     cursor_off();
 
     len = (int)STRLEN(showcmd_buf);