Delaying the runtime.throw until here will print more information.
In particular it will print the signal and code values, which means
it will show the fault address.

The canpanic checks were added recently, in CL 75320043.
They were just not added in exactly the right place.

LGTM=iant
R=dvyukov, iant
CC=golang-codereviews
https://golang.org/cl/83980043