java: Remove unused member variable

Signed-off-by: Pekka Enberg <penberg@kernel.org>
java: Fix sigsegv in jar::load_class

If JAVA_HOME is not specified, for example, _zip will be nullptr in
jar::load_class().

As the user is free to define an arbitrary classpath (where some
components are not files), bail out early in load_class() to avoid
SIGSEGV.

Signed-off-by: Pekka Enberg <penberg@kernel.org>