mac installer - firebird user not created correctly if group already exists

[hmoffatt]

If the firebird group already exists but the user doesn't (which can occur if you use the new Mac transfer process / migration assistant), the installer creates a new user that is has no primary group, which is invalid, and the Firebird service won't start.

This occurs because of a bug in the install-script:

1. line 25 determines that the group already exists, so $gid is not set
2. line 58 uses the unset $gid as the primary group id of the user

Now macOS won't use the new user, and you can't "su - firebird" etc to it, until the group id is set.