runtime: prevent garbage collection during hashmap insertion (fix 2)
runtime: prevent garbage collection during hashmap insertion (fix 2)

Fixes issue 5074 in multi-threaded scenarios.

R=golang-dev, daniel.morsing, dave, dvyukov, bradfitz, rsc
CC=golang-dev, remyoudompheng
https://codereview.appspot.com/7916043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go