sUserClassPath may be empty (instead of containing "-Djava.class.path=...")
here, which older versions of Java have apparently silently ignored.  But
Java 10 now fails with JNI_EINVAL (-6).  (Happens e.g. during
CppunitTest_dbaccess_hsqldb_test.)

Change-Id: Ifd13222f0d0ae69547bd1a9fe0fd5eae47917d0a
Reviewed-on: https://gerrit.libreoffice.org/51949Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>