fix "PHP Warning: preg_match(): No ending delimiter '.' found"

the second argument of param_to_global, if set, becomes the first
argument of a call to preg_match(). Unless it's the special value 'int',
it needs to have regex delimiters around it, like '/.*/'

In this case, we're not interested in the value of the add-member
parameter, we only want to check for its existence, so skip validation.

htdocs/caldav.php

@@ -102,7 +102,7 @@ if ( ! ($request->IsPrincipal() || isset($request->collection) || $request->meth
     @ob_flush(); exit(0);
   }
 }
-param_to_global('add_member', '.*', 'add-member');
+param_to_global('add_member', null, 'add-member');
 $add_member = isset($add_member);
 
 try {