runtime: Don't crash when deleting zero-sized key.
runtime: Don't crash when deleting zero-sized key.

Fixes gcc.gnu.org/PR61258.

LGTM=iant
R=iant
CC=gofrontend-dev
https://golang.org/cl/188910043

qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,golang/gofrontend,anlhord/gofrontend