fix migrator in transaction

[goxiaoy]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

go-sql-driver/mysql#1213 (comment)

Fix migrator issue in a transaction

User Case Description

[jinzhu]

does it causing issues? seems we are not reusing the rows?

[goxiaoy]

In the following line

mysql/migrator.go

Line 166 in 8149e87

columns, rowErr := m.DB.Raw(columnTypeSQL, currentDatabase, stmt.Table).Rows()

busy buffer error will be raised if this migration happens in a transactional db instance