RFR JDK-8193802: NullPointerException from JarFileSystem.getVersionMap()

[Xueming Shen]

Hi

Please help review the change for JDK-8193802.
Issue: https://bugs.openjdk.java.net/browse/JDK-8193802
Webrev: http://cr.openjdk.java.net/~sherman/8193802/webrev

To add a "null" check when building mr lookup tables. The use scenario 
here is
that the jar file and the jarfs env are configured to a mr jarfs, but 
the jar file itself
actually does not have the corresponding directory "meta-inf/versions".

Thanks,
Sherman