allocator_may_return_null doesn't seem to be set to 1 when fuzzers are run under MSan

I added a file to the seed corpus and it triggered an unreproducible MSan crash I reported in https://github.com/google/clusterfuzzlite/issues/67#issuecomment-995356807. By running `./infra/helper.py` I noticed that it failed because the allocator was out of memory. Issues like that are kind of ignored in ASan runs where allocator_may_return_null=1 is passed among ASAN_OPTIONS and propagate until real OOMs are hit, which is handled by the REPORT_OOMS setting.