Fix java.lang.StackOverflowError in getSql

getSql was calling itself by checking (!sql).  I assume the original
intention was to create a local sql instance property and set it on the
first call, but return the same instance after that.

All tests pass.

    | Completed 4 integration tests, 0 failed in 2956ms
    | Completed 93 cli tests, 0 failed in 3176753ms
    | Tests PASSED - view reports in /Users/ajz/dev/grails/plugins/grails-database-migration/target/test-reports