runtime: fix new stack barrier check

During a crash showing goroutine stacks of all threads
(with GOTRACEBACK=crash), it can be that f == nil.

Only happens on Solaris; not sure why.

Change-Id: Iee2c394a0cf19fa0a24f6befbc70776b9e42d25a
Reviewed-on: https://go-review.googlesource.com/17110Reviewed-by: Austin Clements <austin@google.com>

src/runtime/traceback.go

@@ -190,7 +190,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	}
 
 	f := findfunc(frame.pc)
-	if f.entry == stackBarrierPC {
+	if f != nil && f.entry == stackBarrierPC {
 		// We got caught in the middle of a stack barrier
 		// (presumably by a signal), so stkbar may be
 		// inconsistent with the barriers on the stack.