Add a default constructor to JValue.

Change-Id: I7abe801b26f5003c480215c1f1ca64dfc07667ed
diff --git a/src/object.h b/src/object.h
index 3ece535..7413bf9 100644
--- a/src/object.h
+++ b/src/object.h
@@ -60,6 +60,9 @@
 typedef PrimitiveArray<int16_t> ShortArray;
 
 union JValue {
+  // We default initialize JValue instances to all-zeros.
+  JValue() : j(0) {}
+
   uint8_t z;
   int8_t b;
   uint16_t c;