revert Prepare change

Author: methane

connection.go

@@ -171,7 +171,9 @@ func (mc *mysqlConn) Prepare(query string) (driver.Stmt, error) {
 	// Send command
 	err := mc.writeCommandPacketStr(comStmtPrepare, query)
 	if err != nil {
-		return nil, err
+		// STMT_PREPARE is safe to retry.  So we can return ErrBadConn here.
+		mc.log(err)
+		return nil, driver.ErrBadConn
 	}
 
 	stmt := &mysqlStmt{