BUG/MAJOR: patterns: fix double free caused by loading strings from files

A null pointer assignment was missing after a free in commit 7148ce6 (MEDIUM:
pattern: Extract the index process from the pat_parse_*() functions), causing
a double free after loading a file of string patterns.

This bug was introduced in 1.5-dev20, no backport is needed.

Thanks to Sander Klein for reporting this bug and providing the config
needed to trigger it.