Fix crash in osx/x64/exec on 10.9 Mavericks

RM #8771

I played it fast and loose with the ENV pointer for the execve call, which was causing a crash on 10.9. I've nulled out RDX (the env ptr).

Verification
- [x] `./msfpayload osx/x64/exec cmd="/usr/bin/id" X > x64_test_id`
- [x] Runs on OSX 10.9
- [x] Runs on OSX < 10.9