make sure Close always removes the runtime finalizer

This commit fixes a bug in {SQLiteConn,SQLiteStmt}.Close that would lead
to the runtime finalizer not being removed if there was an error closing
the connection or statement. This commit also makes it safe to call
SQLiteConn.Close multiple times.

sqlite3.go

@@ -1782,17 +1782,20 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 }
 
 // Close the connection.
-func (c *SQLiteConn) Close() error {
+func (c *SQLiteConn) Close() (err error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.db == nil {
+		return nil // Already closed
+	}
+	runtime.SetFinalizer(c, nil)
 	rv := C.sqlite3_close_v2(c.db)
 	if rv != C.SQLITE_OK {
-		return c.lastError()
+		err = c.lastError()
 	}
 	deleteHandles(c)
-	c.mu.Lock()
 	c.db = nil
-	c.mu.Unlock()
-	runtime.SetFinalizer(c, nil)
-	return nil
+	return err
 }
 
 func (c *SQLiteConn) dbConnOpen() bool {
@@ -1901,16 +1904,18 @@ func (s *SQLiteStmt) Close() error {
 		return nil
 	}
 	s.closed = true
-	if !s.c.dbConnOpen() {
+	conn := s.c
+	stmt := s.s
+	s.c = nil
+	s.s = nil
+	runtime.SetFinalizer(s, nil)
+	if !conn.dbConnOpen() {
 		return errors.New("sqlite statement with already closed database connection")
 	}
-	rv := C.sqlite3_finalize(s.s)
-	s.s = nil
+	rv := C.sqlite3_finalize(stmt)
 	if rv != C.SQLITE_OK {
-		return s.c.lastError()
+		return conn.lastError()
 	}
-	s.c = nil
-	runtime.SetFinalizer(s, nil)
 	return nil
 }