Fix null pointer dereference in auditParse

There is an issue in LogAudit::auditParse where
android::uidToName(uid) crashes with a null pointer dereference.
Include a null check on the value before passing it on.

Bug: 120043607
Test: End-to-end test with syzkaller as per instructions in bug.
Change-Id: Ic0ac5c3003fcd289ec156ce63fbd668413763429