internal/core/adt: do not set error for nil task state

After we add memory management, a node may already
have been cleared before a task is completed. This
is especially the case in auxiliary tools like
dep. We add this change before its implementation
to see that this change has no effect and to
make bisection easier.

Issue #3334
Issue #2850

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I982e33271853b68e8e8f4a1a4ee4325eaec227bf
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1218181
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl

internal/core/adt/tasks.go

@@ -209,7 +209,9 @@ func processComprehension(ctx *OpContext, t *task, mode runMode) {
 
 	err := n.processComprehension(y, 0)
 	t.err = CombineErrors(nil, t.err, err)
-	t.comp.vertex.state.addBottom(err)
+	if t.comp.vertex.state != nil {
+		t.comp.vertex.state.addBottom(err)
+	}
 }
 
 func processDisjunctions(c *OpContext, t *task, mode runMode) {