Fix gofmt error handler on non-file buffer

[qxxt]

When parsing error occur while formatting a non-file buffer. It'll cause a real error because we're trying to pass buffer-file-name which is null to gofmt--process-errors and gofmt--process-errors is trying to get file-name of that value. This commit pass buffer-name to gofmt--process-errors if current buffer is non-file.