pgsql: fix self._conn.close() being called without connection

If an error occurs on connection establishment, self._conn is None so it can't
be closed.

Change-Id: If6fce24cb27aa7090b5165ca4a895c846c40d2dd

tooz/drivers/pgsql.py

@@ -146,7 +146,8 @@ class PostgresLock(locking.Lock):
         try:
             return _lock()
         except Exception:
-            self._conn.close()
+            if self._conn:
+                self._conn.close()
             raise
 
     def release(self):