Fixed compilation with gcc 7.4.

[ipcyborg]

Summary

SUMMARY: Build "Fixed compilation with gcc 7.4."

Purpose of change

After updating to Ubuntu 18.04, the gcc 7.4 failed to compile CDDA:
art->charge_req may be used uninitialized in this function.

Describe the solution

Fixed the build issue by adding check for null. Should not break anything else, right?

Describe alternatives you've considered

Revert the upgrade.

UPDATE: Upgrading to gcc 8.3.0 does not help.

Additional context

Full error message:

         if( art->charge_req != ACR_NULL && ( it->ammo_remaining() < it->ammo_capacity() ||
             ~~~~~^~~~~~~~~~