event: fix data race
event: fix data race

This is definitely the best fix, but I'm committing this one to make the
build green again. The event monitoring code can definitely take some
love, I'll try to refactor it over the weekend.

ybogdanov/go-dockerclient,csrwng/go-dockerclient,csrwng/go-dockerclient,davidstack/go-dockerclient,ybogdanov/go-dockerclient,fsouza/go-dockerclient,tarsisazevedo/go-dockerclient,davidstack/go-dockerclient,paulbellamy/go-dockerclient,tarsisazevedo/go-dockerclient