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

apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go