[Asan] Fix -Wunused-private-field in non-assertion builds (NFC)

llvm-project/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp:650:13:
error: private field 'OwnerFn' is not used [-Werror,-Wunused-private-field]
  Function *OwnerFn = nullptr;
            ^
1 error generated.