Fix (Sub-)Value assignment

[ylavic]

Extracted from PR #1485, should probably live on its own...

The issue is, from valuetest.cpp::MergeDuplicateKey():

        for (Value::MemberIterator itr = v.MemberBegin(); itr != v.MemberEnd(); ++itr) {
            if (itr->value.Size() == 1)
                itr->value = itr->value[0];
            ...
        }

Without the fix, in current GenericValue& operator=(GenericValue&), itr->value.~GenericValue() gets called before itr->value.RawAssign(itr->value[0]) and thus itr->value[0] may be used after free (with a kneedFree allocator).

PS: the VC2013 compilation fix for sortkeys.cpp is also included here to pass ci.

[coveralls]

Coverage increased (+0.0001%) to 99.729% when pulling 117276c on ylavic:sub_value_assignment into 49aa0fc on Tencent:master.

[miloyip]

I suggest adding an unit test for testing this case.

[ylavic]

I suggest adding an unit test for testing this case.

Done.