BUG/MAJOR: lru: fix unconditional call to free due to unexpected semi-colon

BUG/MAJOR: lru: fix unconditional call to free due to unexpected semi-colon

Dmitry Sivachenko reported the following build warning using Clang, which
is a real bug :

  src/lru.c:133:32: warning: if statement has empty body [-Wempty-body]
                                  if (old->data && old->free);
                                                             ^
It results in calling old->free(old->data) even when old->free is NULL,
hence crashing on cached patterns.

The same bug appears a few lines below in lru64_destroy() :

  src/lru.c:195:33: warning: if statement has empty body [-Wempty-body]
                          if (elem->data && elem->free);
                                                       ^
Both were introduced in 1.6-dev2 with commit f90ac55 ("MINOR: lru: Add the
possibility to free data when an item is removed"), so no backport is needed.