efi_loader: fix unreachable statement in efi_sigstore_parse_siglist

"if (left < esl->signature_size)" is not reachable in a while loop.
But it is still valuable in case that a given signature database is
somehow corrupted. So fix the while loop condition.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>