[release-branch.go1.1] time: prevent a panic from leaving the timer mutex held

««« CL 10373047 / 974a69ed9fcf
time: prevent a panic from leaving the timer mutex held

When deleting a timer, a panic due to nil deref
would leave a lock held, possibly leading to a deadlock
in a defer. Instead return false on a nil timer.

Fixes #5745.

R=golang-dev, daniel.morsing, dvyukov, rsc, iant
CC=golang-dev
https://golang.org/cl/10373047

»»»

Update #5928

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11666046