test: fix bufferevent/bufferevent_pair_release_lock for freebsd

test: fix bufferevent/bufferevent_pair_release_lock for freebsd

On FreeBSD with kqueue there is a call to evthread_debug_lock_mark_unlocked()
during event_base_free(), that will fail with an assert because of unmatched
"held_by", fix this by reseting lock callbacks to NULL before
event_base_free().

Trace:
  bufferevent/bufferevent_pair_release_lock: [warn] Trying to disable lock functions after they have been set up will probaby not work.
  [warn] Trying to disable lock functions after they have been set up will probaby not work.

    FAIL libevent/test/regress_bufferevent.c:259: lock: lock error[err] libevent/evthread.c:277: Assertion lock->held_by == me failed in evthread_debug_lock_mark_unlocked
  [New Thread 802006400 (LWP 100070/regress)]

  Program received signal SIGABRT, Aborted.
  [Switching to Thread 802006400 (LWP 100070/regress)]
  0x000000080167d6ca in thr_kill () from /lib/libc.so.7
  (gdb) bt
  #0  0x000000080167d6ca in thr_kill () from /lib/libc.so.7
  #1  0x0000000801752149 in abort () from /lib/libc.so.7
  #2  0x00000000004dff44 in event_exit (errcode=-559030611) at libevent/log.c:105
  #3  0x00000000004e053c in event_errx (eval=-559030611, fmt=0x5182cc "%s:%d: Assertion %s failed in %s") at libevent/log.c:162
  #4  0x00000000004d9954 in evthread_debug_lock_mark_unlocked (mode=0, lock=0x802017060) at libevent/evthread.c:277
  #5  0x00000000004d909a in debug_lock_unlock (mode=0, lock_=0x802017060) at libevent/evthread.c:290
  #6  0x00000000004e132c in evsig_dealloc_ (base=0x80201e300) at libevent/signal.c:434
  #7  0x00000000004e36c1 in kq_dealloc (base=0x80201e300) at libevent/kqueue.c:435
  #8  0x00000000004c9a44 in event_base_free_ (base=0x80201e300, run_finalizers=1) at libevent/event.c:855
  #9  0x00000000004c931a in event_base_free (base=0x0) at libevent/event.c:887
  #10 0x0000000000452657 in lock_unlock_free_thread_cbs () at libevent/test/regress_bufferevent.c:279
  #11 0x0000000000452621 in free_lock_unlock_profiler (data=0x8020170a0) at libevent/test/regress_bufferevent.c:317
  #12 0x000000000044bc8f in test_bufferevent_pair_release_lock (arg=0x8020170a0) at libevent/test/regress_bufferevent.c:334
  #13 0x00000000004b2288 in testcase_run_bare_ (testcase=0x737660) at libevent/test/tinytest.c:105
  #14 0x00000000004b1e72 in testcase_run_one (group=0x738c90, testcase=0x737660) at libevent/test/tinytest.c:252
  #15 0x00000000004b2930 in tinytest_main (c=3, v=0x7fffffffead0, groups=0x738c20) at libevent/test/tinytest.c:434
  #16 0x00000000004982fe in main (argc=3, argv=0x7fffffffead0) at libevent/test/regress_main.c:459
  (gdb) f 4
  #4  0x00000000004d9954 in evthread_debug_lock_mark_unlocked (mode=0, lock=0x802017060) at libevent/evthread.c:277
  277                     EVUTIL_ASSERT(lock->held_by == me);
  Current language:  auto; currently minimal
  (gdb) p lock
  $1 = (struct debug_lock *) 0x802017060
  (gdb) p lock->held_by
  $2 = 0
  (gdb) p me
  $3 = 34393318400