dcache-xroot: restore proper handling of single-slash paths

Motivation:

https://rb.dcache.org/r/13923/
master@c491f2d312abcbadefcf1df2af85c5620e1e1867

introduced a regression which fails URLs like:

`xroot://example.org:1094/foo`

(single slash separating host from path).

Modification:

Catch the exception from `FsPath.create` and restore the
missing slash in that case.

Result:

Proper behavior restored.

Target: master
Patch: https://rb.dcache.org/r/13962/
Requires-notes:  no (unreleased change)
Acked-by: Tigran

modules/dcache-xrootd/src/main/java/org/dcache/xrootd/door/XrootdRedirectHandler.java

@@ -1379,10 +1379,17 @@ private FsPath createFullPath(String path, Map<String, String> opaque)
           throws PermissionDeniedCacheException {
         String fromOpaque = opaque.get(EFFECTIVE_ROOT_NAME);
         FsPath root = fromOpaque != null ? FsPath.create(fromOpaque) : effectiveRoot();
+
+        if (path.charAt(0) != '/') {
+            path = "/" + path;
+        }
+
         FsPath fullPath = FsPath.create(path);
+
         if (fullPath.hasPrefix(root)) {
             path = fullPath.stripPrefix(root);
         }
+
         return root.chroot(path);
     }