RFR 8252754: Hash code calculation of JfrStackTrace is inconsistent

[Jaroslav Bachorík]

Please, review this small patch to bring back consistency between
JfrStackTrace::record_safe and JfrStackTrace::record_thread stack
trace hashcode calculation.

JIRA: https://bugs.openjdk.java.net/browse/JDK-8252754
Webrev: http://cr.openjdk.java.net/~jbachorik/8252754/dev/webrev/

Tested with tier1 and JFR tests and all expected tests are passing.

Cheers,

-JB-