branches/zip: When assigning lock->recursive = FALSE, also flag

lock->writer_thread invalid, so that Valgrind will catch more errors.
This is related to Issue #175.

include/sync0rw.ic

@@ -574,6 +574,8 @@ rw_lock_x_unlock_func(
 	if (lock->lock_word == 0) {
 		/* Last caller in a possible recursive chain. */
 		lock->recursive = FALSE;
+		UNIV_MEM_INVALID(&lock->writer_thread,
+				 sizeof lock->writer_thread);
 	}
 
 #ifdef UNIV_SYNC_DEBUG
@@ -618,6 +620,8 @@ rw_lock_x_unlock_direct(
 
 	if (lock->lock_word == 0) {
 		lock->recursive = FALSE;
+		UNIV_MEM_INVALID(&lock->writer_thread,
+				 sizeof lock->writer_thread);
 	}
 
 	lock->lock_word += X_LOCK_DECR;