RFR(xs): 8156789: missing condition in ClassPathZipEntry::open_versioned_entry()

[Calvin Cheung]

This change is for adding back the !_is_boot_append condition back to 
ClassPathZipEntry::open_versioned_entry().
It was removed inadvertently during a bug fix in another repo.

bug: https://bugs.openjdk.java.net/browse/JDK-8156789
webrev: http://cr.openjdk.java.net/~ccheung/8156789/webrev.00/

Testing:
     JPRT fastdebug builds.
     RBT on --test hotspot/test/:hotspot_runtime

thanks,
Calvin