x86/setup: Avoid OoB E820 lookup when calculating the L1TF safe address

x86/setup: Avoid OoB E820 lookup when calculating the L1TF safe address

A number of corner cases (most obviously, no-real-mode and no Multiboot memory
map) can end up with e820_raw.nr_map being 0, at which point the L1TF
calculation will underflow.

Spotted by Coverity.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
master commit: 3e4ec07e14bce81f6ae22c31ff1302d1f297a226
master date: 2018-08-16 18:10:07 +0100