sync: don't keep func alive after OnceFunc panics
This moves the f = nil assignment to the defer statement,
so that in case the functions panics, the f func is not
referenced anymore.
Change-Id: I3e53b90a10f21741e26602270822c8a75679f163
GitHub-Last-Rev:
bda01100c6d48d1b0ca3e1baefef4d592cca1fee
GitHub-Pull-Request: golang/go#68636
Reviewed-on: https://go-review.googlesource.com/c/go/+/601240
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>