fix a strange failure during testing where a call to registerNs
gives an error about cannot *unregister*.

Thanks to Don Armstrong for providing the idea
---
 lib/Biber/Input/file/biblatexml.pm |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- biber.orig/lib/Biber/Input/file/biblatexml.pm
+++ biber/lib/Biber/Input/file/biblatexml.pm
@@ -173,7 +173,8 @@
   $xml = NFD(decode('UTF-8', $xml));# Unicode NFD boundary
   my $bltxml = XML::LibXML->load_xml(string => $xml);
   my $xpc = XML::LibXML::XPathContext->new($bltxml);
-  $xpc->registerNs($NS, $BIBLATEXML_NAMESPACE_URI);
+  my $tmp = $BIBLATEXML_NAMESPACE_URI;
+  $xpc->registerNs($NS, $tmp);
 
   if ($section->is_allkeys) {
     $logger->debug("All citekeys will be used for section '$secnum'");
