evthread: fix evthread_setup_global_lock_() for debug-lock with a real-lock case

Sample bt:
  (gdb) bt
  #0  0x00000000004a95d2 in evthread_setup_global_lock_ (lock_=0x0, locktype=0, enable_locks=1)
  #1  0x00000000004afbc7 in evsig_global_setup_locks_ (enable_locks=1) at signal.c:475
  #2  0x00000000004a5da8 in event_global_setup_locks_ (enable_locks=1) at event.c:3838
  #3  0x00000000004a8983 in evthread_set_lock_callbacks (cbs=0x7fffffffdde0) at evthread.c:129
  #4  0x0000000000445a8f in use_lock_unlock_profiler () at regress_bufferevent.c:306
  #5  0x0000000000445af2 in test_bufferevent_pair_release_lock (arg=0x70e710) at regress_bufferevent.c:327
  #6  0x000000000048c9a8 in testcase_run_bare_ (testcase=0x6fdc10 <bufferevent_testcases+80>) at tinytest.c:105
  #7  0x000000000048cc8a in testcase_run_one (group=0x6ff270 <testgroups+112>, testcase=0x6fdc10 <bufferevent_testcases+80>) at tinytest.c:252
  #8  0x000000000048d5c9 in tinytest_main (c=3, v=0x7fffffffe008, groups=0x6ff200 <testgroups>) at tinytest.c:434
  #9  0x000000000047a08f in main (argc=3, argv=0x7fffffffe008) at regress_main.c:459

Found-with: regress bufferevent/bufferevent_pair_release_lock (-DEVENT__DISABLE_DEBUG_MODE=ON)

1 files changed, 8 insertions, 7 deletions

diff --git a/evthread.c b/evthread.c
index b8c5fb63..f3f1eddc 100644
--- a/evthread.c
+++ b/evthread.c
@@ -401,17 +401,18 @@ evthread_setup_global_lock_(void *lock_, unsigned locktype, int enable_locks)
 		return evthread_lock_fns_.alloc(locktype);
 	} else {
 		/* Case 4: Fill in a debug lock with a real lock */
-		struct debug_lock *lock = lock_;
+		struct debug_lock *lock = lock_ ? lock_ : debug_lock_alloc(locktype);
 		EVUTIL_ASSERT(enable_locks &&
 		              evthread_lock_debugging_enabled_);
 		EVUTIL_ASSERT(lock->locktype == locktype);
-		EVUTIL_ASSERT(lock->lock == NULL);
-		lock->lock = original_lock_fns_.alloc(
-			locktype|EVTHREAD_LOCKTYPE_RECURSIVE);
 		if (!lock->lock) {
-			lock->count = -200;
-			mm_free(lock);
-			return NULL;
+			lock->lock = original_lock_fns_.alloc(
+				locktype|EVTHREAD_LOCKTYPE_RECURSIVE);
+			if (!lock->lock) {
+				lock->count = -200;
+				mm_free(lock);
+				return NULL;
+			}
 		}
 		return lock;
 	}