[compiler-rt] [rtsan] Fix sanitizer_common/TestCases/dlsym_alloc.c (PR #216579)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

DlsymAlloc::Free check for null is OK, but then we need to make  sure IsMine is as well returns true for nullptr?

https://github.com/llvm/llvm-project/pull/216579