Fix static analyzer warnings

The static analyzer was complaining that we were potentially leaking
memory here (in `ASSERT_NE(ptr, nullptr)` after `new (char)`). This
wasn't correct, but it's also not possible for `new` to return nullptr
without std::nothrow.

In any case, swap to direct calls to `::operator new`, since it looks
like this test explicitly wants calls to `::operator new` to be emitted
(which the C++ standard doesn't guarantee for all `new` expressions).

Bug: 27101951
Test: mma; static analyzer warnings are gone. Also ran
memunreachable_test on marlin; no failures.
Change-Id: Ia740e41079f263040da978ba1ccc71c9c39f53fd