RFR(XS): 8214707: Prevent GCC 8 from reporting error in ClassLoader::file_name_for_class_name()

[Dmitry Chuyko]

Hello,

Please review a small fix for an error reported by GCC 8. It's a false 
positive and minor code clean-up silences the compiler: suffix length is 
extracted into a variable.

bug: https://bugs.openjdk.java.net/browse/JDK-8214707
webrev: http://cr.openjdk.java.net/~dchuyko/8214707/webrev.00/
testing: submit repo (mach5-one-dchuyko-JDK-8214707-20181206-1906-13407: 
PASSED)

-Dmitry