go vet warning: self-assignment of sql to sql

[egonelbre]

go vet indicates there's a mistake related to the code in https://github.com/googleapis/go-sql-spanner/blob/6c5c8cb70166ae0abe0f8b2c764cf1970f8cab96/statement_parser.go#L298

$ go vet ./...
# github.com/googleapis/go-sql-spanner
# [github.com/googleapis/go-sql-spanner]
./statement_parser.go:298:3: self-assignment of sql to sql

This also indicates that there's no linting setup for the project. Usually the good starting options are go vet and staticcheck, because they have the least amount of noise.

~PS: I also noticed that the tests are running without -race, which may hide some bugs as well.~