[syslinux] request backport fix for isolinux 4.xx branch

[Ady]

Recently a patch by HPA was added to the elflink branch,
"isolinux: Clear upper half of EDX before using..."
http://git.zytor.com/?p=syslinux/syslinux.git;a=commit;h=870b84dd8714d
dfccc9288025331423efa6d76b7

The patch was then applied to the firmware branch too.

The patch solves an issue introduced by a prior commit "isolinux: 
Update LBA in getlinsec loop".

Since the original problematic commit was also originally applied to 
the 4.xx branch
http://git.zytor.com/?p=syslinux/syslinux.git;a=commit;h=cb015497a4e43
3ba81a47b28790b325807185617

and being 4.xx still more stable / predictable and more 
feature-complete than 6.xx for BIOS systems, I'd like to kindly 
request for the patch to be applied to the 4.xx branch too.

TIA,
Ady.