Make DynArray support objects larger than 1 gigabyte

The expression ``const int newAllocated = cap * 2;`` easily causes
overflow, as soon as the input is 1.0 GiB. This goes unnoticed because
release builds of tinyxml2 do not have active assertions.

The change in commit 9.0.0-20-g8fd6cc6 did not do anything useful;
the signed multiplication overflow (and thus undefined behavior)
still occurs.

Using ``int`` in this class is really archaic, because it limits the
class to a gigabyte even on 64-bit platforms.

The multiplication overflow check also needs to include sizeof(T),
otherwise you can run into unsigned multiplication overflow (defined,
but undesirable) in the memcpy() call.

testcase:

int main()
{
        tinyxml2::XMLDocument doc;
        doc.InsertEndChild(doc.NewDeclaration());
        auto root = doc.NewElement("root");
        size_t sz = 0x80000001;
        auto blank = new char[sz];
        memset(blank, ' ', sz);
        blank[sz-1]='\0';
        root->SetText(blank);
        doc.InsertEndChild(root);
        tinyxml2::XMLPrinter printer(nullptr);
        doc.Print(&printer);
}

tinyxml2.cpp

@@ -2221,7 +2221,7 @@ void XMLDocument::MarkInUse(const XMLNode* const node)
 	TIXMLASSERT(node);
 	TIXMLASSERT(node->_parent == 0);
 
-	for (int i = 0; i < _unlinked.Size(); ++i) {
+	for (size_t i = 0; i < _unlinked.Size(); ++i) {
 		if (node == _unlinked[i]) {
 			_unlinked.SwapRemove(i);
 			break;