runtime: SIGSEGV in scanstack

[adonovan]

#!watchflakes
post <- `SIGSEGV: segmentation violation` && builder ~ `netbsd-arm.*`

Using go at eb57c09, this log from the netbsd-arm64-bsiegert builder at shows a SIGSEGV in the stack iterator in GOROOT/src/runtime/traceback.go:

func (u *unwinder) next() {
	frame := &u.frame
	f := frame.fn
	gp := u.g.ptr()

	// Do not unwind past the bottom of the stack.
	if frame.lr == 0 {
		u.finishInternal()
		return
	}
	flr := findfunc(frame.lr)
	if !flr.valid() {
		// This happens if you get a profiling interrupt at just the wrong time.
		// In that context it is okay to stop early.
		// But if no error flags are set, we're doing a garbage collection and must
		// get everything, so crash loudly.
		fail := u.flags&(unwindPrintErrors|unwindSilentErrors) == 0
		doPrint := u.flags&unwindSilentErrors == 0
		if doPrint && gp.m.incgo && f.funcID == funcID_sigpanic {   // <---------- SIGSEGV on this line
			// We can inject sigpanic
			// calls directly into C code,
			// in which case we'll see a C
			// return PC. Don't complain.
			doPrint = false
		}

[mknyszek]

CC @golang/netbsd

Maybe CC @aclements? We should see if this has happened before Austin's traceback changes.

[gopherbot]

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)