Fix toJsonClassLoaderContextRec size bug

toJsonClassLoaderContextRec's size is twice as large than expected. And
the initial values is filled with null value.

Bug: 158843648
Test: m nothing(testcase is added)
Change-Id: I24c23269bd16baa18481f34b85c543d41f26d0e0
diff --git a/dexpreopt/class_loader_context.go b/dexpreopt/class_loader_context.go
index ad52b00..f76a205 100644
--- a/dexpreopt/class_loader_context.go
+++ b/dexpreopt/class_loader_context.go
@@ -544,13 +544,13 @@
 // Recursive helper for toJsonClassLoaderContext.
 func toJsonClassLoaderContextRec(clcs []*ClassLoaderContext) []*jsonClassLoaderContext {
 	jClcs := make([]*jsonClassLoaderContext, len(clcs))
-	for _, clc := range clcs {
-		jClcs = append(jClcs, &jsonClassLoaderContext{
+	for i, clc := range clcs {
+		jClcs[i] = &jsonClassLoaderContext{
 			Name:        clc.Name,
 			Host:        clc.Host.String(),
 			Device:      clc.Device,
 			Subcontexts: toJsonClassLoaderContextRec(clc.Subcontexts),
-		})
+		}
 	}
 	return jClcs
 }