refactor: fix govet warning

[fivecut]

What does this pull request do? Explain your changes. (required)

fix the go vet ./... warning:

# github.com/livepeer/go-livepeer/trickle
trickle/trickle_subscriber.go:145:4: the cancel function is not used on all paths (possible context leak)
trickle/trickle_subscriber.go:140:4: this return statement may be reached without using the cancel var defined on line 145

Specific updates (required)

How did you test each of these updates (required)

Does this pull request close any open issues?

Checklist:

• [ ] Read the contribution guide
• [ ] make runs successfully
• [ ] All tests in ./test.sh pass
• [ ] README and other documentation updated
• [ ] Pending changelog updated