don't leave empty cidFile behind

This makes `--cidfile` clean up empty container ID files. These are
left behind when creating the container fails.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4621 from unclejack/fix_empty_cidfile

don't leave empty CIDFile behind