Initialize data members in constructors instead of direct initialize in
class definition, which is a C++11 extension.

Change-Id: I477fe484d737f4c60c519d68f431940ffaa33431

src/google/protobuf/map_entry_lite.h

diff --git a/src/google/protobuf/map_entry_lite.h b/src/google/protobuf/map_entry_lite.h
index 0602d84..304fba8 100644
--- a/src/google/protobuf/map_entry_lite.h
+++ b/src/google/protobuf/map_entry_lite.h
@@ -359,7 +359,7 @@
     typedef void DestructorSkippable_;
   };
 
-  MapEntryLite() : arena_(NULL) {
+  MapEntryLite() : default_instance_(NULL), arena_(NULL) {
     KeyCppHandler::Initialize(&key_, NULL);
     ValueCppHandler::InitializeMaybeByDefaultEnum(
         &value_, default_enum_value, NULL);
@@ -367,7 +367,7 @@
   }
 
   explicit MapEntryLite(Arena* arena)
-      : arena_(arena) {
+      : default_instance_(NULL), arena_(arena) {
     KeyCppHandler::Initialize(&key_, arena);
     ValueCppHandler::InitializeMaybeByDefaultEnum(
         &value_, default_enum_value, arena);
@@ -382,7 +382,7 @@
     default_instance_ = default_instance;
   }
 
-  MapEntryLite* default_instance_ = NULL;
+  MapEntryLite* default_instance_;
 
   KeyBase key_;
   ValueBase value_;