BUG/MEDIUM: checks: segfault with external checks in a backend section

BUG/MEDIUM: checks: segfault with external checks in a backend section

The documentation indicates that external checks can be used in a backend
section, but the code requires a listener to provide information in the script
arguments.
External checks were initialized lately, during the first check, leaving some
variables uninitialized in such a scenario, which trigger the segfault when
accessed to collect errors information.

To prevent the segfault, currently we should initialize the external checks
earlier, during the process initialiation itself and quit if the error occurs.

This fix is specific to the 1.6 branch.