Unhandled error in select.go causes a nil pointer dereference

In `github.com/upper/db/v4@v4.7.0/internal/sqlbuilder/select.go:428` a possible error is ignored.

```go
func (sel *selector) statement() *exql.Statement {
	sq, _ := sel.build()
	return sq.statement()
}
```

If an error takes place, `sq` can be `nil`, resulting into a panic "runtime error: invalid memory address or nil pointer dereference".