mem: Add invalid context id check on LLSC checks

mem: Add invalid context id check on LLSC checks

If the request's address is in the LLSC list, its context Id was being
fetched unconditionally, which could cause the assert at
Request::contextId() to fail.

Change-Id: Iae9791f81c8fe9a7fcd842cd8ab7db18f34f2808
Signed-off-by: Tiago Muck <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18792
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>