RFR: 8138971: G1CollectorPolicy::_rs_lengths_prediction is not initialized before use

[Erik Helin]

Hi all,

this tiny patch initializes the variable
G1CollectorPolicy::_rs_lengths_prediction in the constuctor for
G1CollectorPolicy.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8138971

Webrev:
http://cr.openjdk.java.net/~ehelin/8138971/webrev.00/

Testing:
JPRT

Thanks,
Erik