For return value of local_gpa2hpa, either INVALID_HPA or NULL
means the EPT walking failure. Current code only take care of
NULL return and leave INVALID_HPA as correct case.
In some cases (if guest page table is filled with invalid memory
address), it could crash ACRN from guest.
Add INVALID_HPA return check as well.
Also add @pre assumptions for some gpa2hpa usages.
Tracked-On: #4730
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>