[compiler-rt] [compiler-rt][nfc] Return size 0 explicitly when both pointers are NULL (PR #81789)

[Mingming Liu via llvm-commits]

minglotus-6 wrote:

I'll abandon this patch, given the pattern `if(Ptr)` is used and valid. Thanks for the offline discussion!

https://github.com/llvm/llvm-project/pull/81789