Crash if unused emit native

[samphunter]

Issue description:

Using emit sysreq.c native; without using the native elsewhere in the code will crash compiler.

Minimal complete verifiable example (MCVE):

native print(const str[]);

main() {
    new str[12];
    emit {
        push.adr str
        push.c 4
        sysreq.c print
        stack 8
    }
}

Workspace Information:

• Compiler version: 3.10.9
• Command line arguments provided (or sampctl version): -;+ -)+
• Operating System: Windows 8.1

[Daniel-Cortez]

I think I fixed this bug in a local repo back in December, as part of a batch of fixes for the __emit operator. Now I'm just waiting until #393 is merged before making my own PR, in order to avoid any further conflicts.

[Daniel-Cortez]

Fixed in #421.