RFR 8240324: Improve is_boot_class_loader_data() by adding simple check

[Harold Seigel]

Hi,

Please review this trivial fix for JDK-8240324 to potentially avoid 
accessing the class loader oop.

Open Webrev: http://cr.openjdk.java.net/~hseigel/bug_8240324/webrev/

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8240324

The fix was regression tested by running Mach5 tiers 1 and 2 tests and 
builds on Linux-x64, Solaris, Windows, and Mac OS X, by running Mach5 
tiers 3-5 tests on Linux-x64, and JCK lang and VM tests on Linux-x64.

Thanks, Harold