ParseXMLResponse panics if empty XML is provided

[symtor]

In version 0.4.9 and 0.4.10:

Regression added via commit https://github.com/crewjam/saml/commit/aee3fb1edeeaf1088fcb458727e0fd863d277f8b : When I call ParseXMLResponse(decodedResponseXML []byte, possibleRequestIDs []string) with an empty document ([]byte{}), doc.Root() is nil but gets passed into parseResponse(). No further input validation is performed, so there's a panic when the code attempts to access the node's data.