Avoid reading uninitialized variables in serialization code (triggered when running "make check") #14757

Avoid reading uninitialized variables in serialization code (triggered when running make check).

Note that the variable nVersionDummy is read in the call to the CVarInt constructor:

template<VarIntMode Mode, typename I>
class CVarInt
{
protected:
    I &n;
public:
    explicit CVarInt(I& nIn) : n(nIn) { }
…
}

template<typename T>
inline T& REF(const T& val)
{
    return const_cast<T&>(val);
}

CVarInt<Mode, I> WrapVarInt(I& n) { return CVarInt<Mode, I>{n}; }

#define VARINT(obj, ...) WrapVarInt<__VA_ARGS__>(REF(obj))

unsigned int nVersionDummy;
::Unserialize(s, VARINT(nVersionDummy));

Sorry! My bad: It is passed as a reference all the way down to CVarInt, so we're not reading it :-)