JDK-8329314 showed that verification is rather slow in G1; one optimization in JDK-8329570 removed unnecessary check from the per-reference loop.

It makes sense to actually handle null references first, even before asserts because
- they are very common, and there is nothing to do for them at all
- even the number of failure check should be afterwards because null references do not add to failures

This change, for that particular "benchmark" reduces VM execution time from 3s (~3.9s cpu time) to 2.2s (~3.1s cpu time).