command: Fix formatting for empty usage string

Introduced by requirement in 54d6330b.

Change-Id: If3dba057127b54b15ca7f364f37c6286d34f77e0
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Reviewed-on: http://openocd.zylin.com/1858
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/helper/command.c

@@ -868,7 +868,7 @@ static COMMAND_HELPER(command_help_show, struct command *c, unsigned n,
 	free(cmd_name);
 
 	if (is_match) {
-		if (c->usage) {
+		if (c->usage && strlen(c->usage) > 0) {
 			LOG_USER_N(" ");
 			command_help_show_wrap(c->usage, 0, n + 5);
 		} else