#58 Raise error on double hyphen in comment.

[JHS]

TinyXML does not flag an XML document that is malformed because of double hyphens occuring inside comments as erroneous.

Example of such a document:

<!--
<paren>
<!-- <child> -->
</paren>
-->

One comes across this bug when (mistakenly) trying to nest comments.

The attached patch (against git commit 94b1760fb) fixes this and adds a test for it.

[JHS]