RFR8194230, jdk/internal/jrtfs/remote/RemoteRuntimeImageTest.java fails with NPE

[Felix Yang]

Hi all,

     please review a patch to improve the checking on the settings. 
Originally it will throw NPE, if specified path is invalid.

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8194230

Webrev:

     http://cr.openjdk.java.net/~xiaofeya/8194230/webrev.00/

Thanks,

Felix