cmd/trace: traces may end with pending mark assists

There's no guarantee that all in-progress mark assists will finish
before the trace does. Don't crash if that happens.

I haven't added a test because there's quite a bit of ceremony involved
and the bug is fairly straightforward.

Change-Id: Ia1369a8e2260fc6a328ad204a1eab1063d2e2c90
Reviewed-on: https://go-review.googlesource.com/37540Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/trace/trace.go

@@ -475,7 +475,7 @@ func generateTrace(params *traceParams) (ViewerData, error) {
 			goFinish := getGInfo(ev.G).start.Link
 			fakeMarkStart := *ev
 			text := "MARK ASSIST"
-			if markFinish.Ts > goFinish.Ts {
+			if markFinish == nil || markFinish.Ts > goFinish.Ts {
 				fakeMarkStart.Link = goFinish
 				text = "MARK ASSIST (unfinished)"
 			}
@@ -496,7 +496,7 @@ func generateTrace(params *traceParams) (ViewerData, error) {
 				goFinish := ev.Link
 				fakeMarkStart := *ev
 				text := "MARK ASSIST (resumed, unfinished)"
-				if markFinish.Ts < goFinish.Ts {
+				if markFinish != nil && markFinish.Ts < goFinish.Ts {
 					fakeMarkStart.Link = markFinish
 					text = "MARK ASSIST (resumed)"
 				}