[libcxxabi] Fix NO_THREADS version of test_exception_storage.pass.cpp

`thread_code` returns param, which for NO_THREADS is going to be
`&thread_globals`. Thus, the return value will never be null. The test
was probably meant to check if `*thread_code(&thread_globals) == 0`.
However, to avoid the extra cast, and to bring the NO_THREADS version
more in line with the regular version of the test, this changes it to
check if thread_globals == 0 directly.

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D113048