Fix race in SQLiteStmt.Close by holding conn lock across cache check

sqlite3.go

@@ -1970,6 +1970,10 @@ func (c *SQLiteConn) putCachedStmt(s *SQLiteStmt) bool {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
+	return c.putCachedStmtLocked(s)
+}
+
+func (c *SQLiteConn) putCachedStmtLocked(s *SQLiteStmt) bool {
 	if c.db == nil {
 		return false
 	}
@@ -2164,12 +2168,20 @@ func (s *SQLiteStmt) Close() error {
 		s.c = nil
 		return nil
 	}
-	if !conn.dbConnOpen() {
+	if s.cacheKey != "" {
+		conn.mu.Lock()
+		if conn.db == nil {
+			conn.mu.Unlock()
 			return errors.New("sqlite statement with already closed database connection")
 		}
-	if s.cacheKey != "" && conn.putCachedStmt(s) {
+		if conn.putCachedStmtLocked(s) {
+			conn.mu.Unlock()
 			return nil
 		}
+		conn.mu.Unlock()
+	} else if !conn.dbConnOpen() {
+		return errors.New("sqlite statement with already closed database connection")
+	}
 	s.s = nil
 	s.c = nil
 	rv := C.sqlite3_finalize(stmt)