mozilla/rr — pull request #948 by cgjones (2014-03-19 04:40:01+00:00)
Title: Don't hang if reading an un-started counter.

Resolves #947.

I tried to write a test, but it looked like it wouldn't economical. The very first event in a thread's trace has to be a time-slice interrupt, and that's hard to reliably set up.