Look for that assertion in any known issues in the splunk docs
Upgrade to the latest patch level in your version (if you are in Splunk 7.1 release train, then 7.1.x where x is the most recent)
Submit a support case
When splunk exits on an assertion failure, this is it saying that a internal constraint in the code has been violated. Usually, these require access to the source code to understand and require a code fix to resolve. If it's not been resolved already, then you will need a support case.