[compiler-rt] [compiler-rt] Guard sanitizer DlsymAlloc::Free from nullptr (PR #217097)

[via llvm-commits]

davidtrevelyan wrote:

I humbly share @cjappl's preference for this pattern, if the alternative requires making `DlsymAlloc::PointerIsMine(nullptr)` return `true` for stable operation. I personally find `DlsymAlloc::PointerIsMine(nullptr)` difficult to predict semantically, because I find the concept of `nullptr` ownership unclear. I don't feel strongly about it, though, and defer to others with more experience in this area for approval! 

https://github.com/llvm/llvm-project/pull/217097