logic error in cups_job_completed

The cupsFreeJobs and job_state check were inside the for loop.
Therefore when the job was found and 'break' issued, the loop would exit
and cupsFreeJob and the job_state check would never get called.
Silence clang warning about const char*

URL is later assigned from cupsServer(), which returns const char*.
printer_exists is missing return statement

I'm guessing that it only functioned due to how gcc compiled it.  When
compiled with clang, renders library unusable due to every call
raising "The printer or destination doesn't exist!"
Merge pull request #8 from nathanstitt/clang_fixes

Clang fixes