[asan] Fix null_deref test with zero base shadow.

With zero base shadow, shadow for near-zero access is itself at near-zero
location. As a result, this test crashes on the shadow access, and not on the
app access.

Relax a check to match this behavior.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201324 91177308-0d34-0410-b5e6-96231b3b80d8