RFR: 8044671: NPE from JapaneseEra when a new era is defined in calendar.properties

[dmeetry degrave]

Hello,

Please review a simple fix (jdk 8 and 9) for

8044671: NPE from JapaneseEra when a new era is defined in 
calendar.properties

bug: https://bugs.openjdk.java.net/browse/JDK-8044671
fix: http://cr.openjdk.java.net/~dmeetry/8044671/webrev.01

No automated regression test, unfortunately, it would require to modify 
"private static final Era[] eras;" in 
java/util/JapaneseImperialCalendar.java, though it's easy to test it by 
calling 'JapaneseDate date = JapaneseDate.of(2100,1,1);' with a new era 
added via JAVA_HOME/jre/lib/calendars.properties (test and 
calendars.properties are attached to the bug).

thanks,
dmeetry