Apache FOP Code fix (bug 46211)

Code fix
by
AD
(fixes bug 46211)

:

Fixed some multi-threading issues in FontCache.java:

remove the unused private readObject()
make the changeLock member final (initialization-safety + impossible to reassign)
perform the HashMap check for a failed font inside the synchronized block to avoid a race condition

Thanks to rogov.AT.devexperts.com.