RFR: Fix negative index OOB exception in string dedup stress test

[Zhengyu Gu]

It is indeed the result of Math.abs(Integer.MIN_VALUE) = 
Integer.MIN_VALUE, which is a negative value (-2147483648).

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/dedup_neg_index/webrev.00/

Test:
   With following script.

for i in {0..50}
do
${JAVA_HOME}/bin/java -Xmx512M -XX:+UnlockExperimentalVMOptions 
-XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions 
-XX:ShenandoahGCHeuristics=passive  -XX:-ShenandoahDegeneratedGC 
-XX:+UseStringDeduplication -D
targetOverwrites=40000000 ShenandoahStrDedupStress
done


Thanks,

-Zhengyu