analyze: use _cleanup_ for struct unit_times

analyze: use _cleanup_ for struct unit_times

This introduces a has_data boolean field in struct unit_files which can
be used to detect the end of the array.

Use a _cleanup_ for struct unit_files in acquire_time_data and its
callers. Code for acquire_time_data is also simplified by replacing
goto's with straight returns.

Tested: By running the commands below, also checking them under valgrind.
  - build/systemd-analyze blame
  - build/systemd-analyze critical-chain
  - build/systemd-analyze plot

Fixes: Coverity finding CID 996464.