IkiWiki::Plugin::openid: as a precaution, do not call non-coderefs

We're running under "use strict" here, so if CGI->param's array-context
misbehaviour passes an extra non-ref parameter, it shouldn't be executed
anyway... but it's as well to be safe.

[commit message added by smcv]

IkiWiki/Plugin/openid.pm

@@ -119,7 +119,9 @@ sub validate ($$$;$) {
 	my $claimed_identity = $csr->claimed_identity($openid_url);
 	if (! $claimed_identity) {
 		if ($errhandler) {
+			if (ref($errhandler) eq 'CODE') {
 				$errhandler->($csr->err);
+			}
 			return 0;
 		}
 		else {