Fixed usage of database file in example .ycm_extra_conf.py.

Checking the path is set is not enough, we should also check the path
exists. This allows people to have a general-purpose
.ycm_extra_conf.conf that automatically loads a database file if exists,
and falls back to the manual flags if not.
Merge pull request #693 from tasn/master

Fixed usage of database file in example .ycm_extra_conf.py.