BUG/MEDIUM: patterns: previous fix was incomplete

Dmitry Sivachenko <trtrmitya@gmail.com> reported that commit 315ec42
("BUG/MEDIUM: pattern: don't load more than once a pattern list.")
relies on an uninitialised variable in the stack. While it used to
work fine during the tests, if the uninitialized variable is non-null,
some patterns may be aggregated if loaded multiple times, resulting in
slower processing, which was the original issue it tried to address.

The fix needs to be backported to 1.5.