core/state/snapshot: fix race condition (#24685)

Fixes three race conditions found through fuzzing by David Theodore

core/state/snapshot/difflayer.go

@@ -258,6 +258,9 @@ func (dl *diffLayer) Root() common.Hash {
 
 // Parent returns the subsequent layer of a diff layer.
 func (dl *diffLayer) Parent() snapshot {
+	dl.lock.RLock()
+	defer dl.lock.RUnlock()
+
 	return dl.parent
 }