runtime: stop traceback at foreign function

This can only happen when profiling and there is foreign code
at the top of the g0 stack but we're not in cgo.
That in turn only happens with the race detector.

Fixes #13568.

Change-Id: I23775132c9c1a3a3aaae191b318539f368adf25e
Reviewed-on: https://go-review.googlesource.com/18322Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/traceback.go

@@ -241,6 +241,11 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		//	stk is the stack containing sp.
 		//	The caller's program counter is lr, unless lr is zero, in which case it is *(uintptr*)sp.
 		f = frame.fn
+		if f.pcsp == 0 {
+			// No frame information, must be external function, like race support.
+			// See golang.org/issue/13568.
+			break
+		}
 
 		// Found an actual function.
 		// Derive frame pointer and link register.