xen/mmu: weaken flush_tlb_other test
xen/mmu: weaken flush_tlb_other test

Impact: fixes crashing bug

There's no particular problem with getting an empty cpu mask,
so just shortcut-return if we get one.

Avoids crash reported by Christophe Saout <christophe@saout.de>

Signed-off-by: Jeremy Fitzhardinge <35cfe9f678a9e9bbdcf2f6e68cd105ce06d149b1@citrix.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas