solver: fix possible panic from error handler

If error is returned from `Save()`, `rck` is nil and
causes nil dereference panic on accessing `TraceFields` when
printing debug logs.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

solver/cachemanager.go

@@ -298,15 +298,20 @@ func (c *cacheManager) LoadWithParents(ctx context.Context, rec *CacheRecord) (r
 	return results, nil
 }
 
-func (c *cacheManager) Save(k *CacheKey, r Result, createdAt time.Time) (rck *ExportableCacheKey, rerr error) {
+func (c *cacheManager) Save(k *CacheKey, r Result, createdAt time.Time) (rck *ExportableCacheKey, err error) {
 	lg := bklog.G(context.TODO()).WithFields(logrus.Fields{
 		"cache_manager": c.id,
 		"op":            "save",
 		"result":        r.ID(),
 		"stack":         bklog.TraceLevelOnlyStack(),
 	})
 	defer func() {
-		lg.WithError(rerr).WithField("return_cachekey", rck.TraceFields()).Trace("cache manager")
+		if err != nil {
+			lg = lg.WithError(err)
+		} else {
+			lg = lg.WithField("return_cachekey", rck.TraceFields())
+		}
+		lg.Trace("cache manager")
 	}()
 
 	c.mu.Lock()