Fix comparison of nodesets to strings

Fix two bugs in xmlXPathNodeValHash which could lead to errors when
comparing nodesets to strings:

- Only use contents of text nodes to compute the hash for element nodes.
  Comments, PIs, and other node types don't affect the string-value and
  must be ignored.
- Reset `string` to NULL for node types other than text.

Reported by Aleksei on the mailing list:

    https://mail.gnome.org/archives/xml/2017-September/msg00016.html