[syslinux] Fix: previous command at boot prompt not cleared

[Dany St-Amant]

Hello,

 In syslinux-6.04-pre, there's a fix for gcc5 about not returning local pointer. (https://repo.or.cz/syslinux.git/commit/8dc6d758b564a1ccc44c3ae11f265d43628219ce). This fix introduce a bug visible in two ways.

After typing a 'bogus' label/kernel at the boot prompt: pressing ENTER on the next prompt result in using 'bogus' again instead of booting the default, pressing TAB on the next prompt result in the completion of 'bogus' instead of completing all.

Regards,
Dany

*Patch*
--- syslinux-6.03/com32/elflink/ldlinux/cli.c.orig	2019-04-22 08:03:41.344301939 -0500
+++ syslinux-6.03/com32/elflink/ldlinux/cli.c	2019-04-22 08:04:49.188877232 -0500
@@ -145,6 +145,7 @@ const char *edit_cmdline(const char *inp
     len = cursor = 0;
     prev_len = 0;
     x = y = 0;
+    cmdline[0] = 0;
 
     /*
      * Before we start messing with the x,y coordinates print 'input'