fix: panic in SerializePerf if context is cancelled

Due to the changes in 541bc9f,
it is now possible for some of the serialize methods to return nil if
the context is cancelled. This can result in a nil reference panic in a
few cases if the caller isn't prepared for that case.

martian/core/node.go

@@ -1000,6 +1000,9 @@ func (self *Node) serializeState(ctx context.Context) *NodeInfo {
 		}
 		forks = append(forks, fork.serializeState(ctx))
 	}
+	if ctx.Err() != nil {
+		return nil
+	}
 	edges := make([]EdgeInfo, 0, len(self.directPrenodes))
 	for _, prenode := range self.directPrenodes {
 		if ctx.Err() != nil {
@@ -1056,6 +1059,9 @@ func (self *Node) serializePerf(ctx context.Context) (*NodePerfInfo, []*VdrEvent
 			return nil, nil
 		}
 		forkSer, vdrKill := fork.serializePerf(ctx)
+		if ctx.Err() != nil {
+			return nil, nil
+		}
 		forks = append(forks, forkSer)
 		if vdrKill != nil && self.call.Kind() != syntax.KindPipeline {
 			storageEvents = append(storageEvents, vdrKill.Events...)

martian/core/pipestance.go

@@ -675,6 +675,9 @@ func (self *Pipestance) SerializePerf(ctx context.Context) []*NodePerfInfo {
 		perf, _ := node.serializePerf(ctx)
 		ser = append(ser, perf)
 	}
+	if ctx.Err() != nil {
+		return nil
+	}
 	util.LogInfo("perform", "Serializing pipestance performance data.")
 	if len(ser) > 0 && ctx.Err() == nil {
 		overallPerf := ser[0]

martian/core/stage.go

@@ -1575,6 +1575,10 @@ func (self *Fork) serializePerf(ctx context.Context) (*ForkPerfInfo, *VDRKillRep
 				return nil, nil
 			}
 			subforkSer, subforkKillReport := subfork.serializePerf(ctx)
+			if ctx.Err() != nil {
+				// If the context has expired, subforkSer will be nil.
+				return nil, nil
+			}
 			stats = append(stats, subforkSer.ForkStats)
 			if subforkKillReport != nil {
 				killReports = append(killReports, subforkKillReport)