fix omission bugs

Author: arnehormann

Diff for: connection.go

@@ -108,7 +108,7 @@ func (mc *mysqlConn) Begin() (driver.Tx, error) {
 		return &mysqlTx{mc}, err
 	}
 	if err == errNoWrite {
-		return driver.ErrBadConn
+		return nil, driver.ErrBadConn
 	}
 	return nil, err
 }

Diff for: statement.go

@@ -46,7 +46,7 @@ func (stmt *mysqlStmt) Exec(args []driver.Value) (driver.Result, error) {
 	err := stmt.writeExecutePacket(args)
 	if err != nil {
 		if err == errNoWrite {
-			return driver.ErrBadConn
+			return nil, driver.ErrBadConn
 		}
 		return nil, err
 	}
@@ -89,7 +89,7 @@ func (stmt *mysqlStmt) Query(args []driver.Value) (driver.Rows, error) {
 	err := stmt.writeExecutePacket(args)
 	if err != nil {
 		if err == errNoWrite {
-			return driver.ErrBadConn
+			return nil, driver.ErrBadConn
 		}
 		return nil, err
 	}