Fix possible NullPointerException in AbstractAutoCreds

Observed below exception while testing Hive token mechanism.

    ```
    Caused by: java.lang.NullPointerException
            at org.apache.storm.common.AbstractAutoCreds.addTokensToUGI(AbstractAutoCreds.java:219) ~[storm-autocreds-1.2.0.3.1.0.0-526.jar:1.2.0.3.1.0.0-526]
            at org.apache.storm.common.AbstractAutoCreds.populateSubject(AbstractAutoCreds.java:118) ~[storm-autocreds-1.2.0.3.1.0.0-526.jar:1.2.0.3.1.0.0-526]
            at org.apache.storm.security.auth.AuthUtils.populateSubject(AuthUtils.java:228) ~[storm-core-1.2.0.3.1.0.0-526.jar:1.2.0.3.1.0.0-526]
            ... 10 more
    2018-01-19 16:23:26.157 o.a.s.util main [ERROR] Halting process: ("Error on initialization")
    ```