7034532: (fs) AssertionError when working directory is UNC

Reviewed-by: forax, mduigou

src/windows/classes/sun/nio/fs/WindowsFileSystem.java

@@ -56,8 +56,9 @@ class WindowsFileSystem
         // parse default directory and check it is absolute
         WindowsPathParser.Result result = WindowsPathParser.parse(dir);
 
-        if (result.type() != WindowsPathType.ABSOLUTE)
-            throw new AssertionError("Default directory must be absolute/non-UNC");
+        if ((result.type() != WindowsPathType.ABSOLUTE) &&
+            (result.type() != WindowsPathType.UNC))
+            throw new AssertionError("Default directory is not an absolute path");
         this.defaultDirectory = result.path();
         this.defaultRoot = result.root();