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)

src/normal.c

@@ -1798,6 +1798,9 @@ display_showcmd(void)
 {
     int	    len;
 
+    if (p_ch == 0)
+	return;
+
     cursor_off();
 
     len = (int)STRLEN(showcmd_buf);