fix dataraces by #102

[t-yuki]

I've found some dataraces induced by #102 and fixed them.
See attachment files for details.
(How can we avoid such data race hell with httprace...?)

(EDIT) I also fixed a nil panic.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

race2.txt
race3.txt
race.txt

[chrisradek]

@t-yuki
Thank you for your contribution! This looks like a good change. I'll follow-up with a PR to make sure we also run tests using the -race argument to hopefully catch some of these issues sooner.