RFR: JDK-8226561: javac throws NullPointerException in Check.checkClassOverrideEqualsAndHash

[Vicente Romero]

Please review the fix for [1] at [2]. This patch fixes a NPE produced at 
a lint warning checking that a class overriding ::equals should override 
::hashCode too. The thing is that if the class is an inexistent symbol 
then the method symbol returned by Types::implementation is null. The 
fix is checking for this null case,

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8226561
[2] http://cr.openjdk.java.net/~vromero/8226561/webrev.00/