Do not read from nil pointer

Addressing:
FFI::NullPointerError: invalid memory read at address=0x00000000000000
	from /opt/rh/ruby193/root/usr/share/gems/gems/ffi-1.0.9/lib/ffi/pointer.rb:35:in `get_string'
	from /opt/rh/ruby193/root/usr/share/gems/gems/ffi-1.0.9/lib/ffi/pointer.rb:35:in `read_string'
	from /opt/rh/ruby193/root/usr/share/gems/gems/openscap-0.2.1/lib/openscap/ds/arf.rb:34:in `html'
	from (irb):4
Raise an exception if the ds_rds_session_get_html_report errors.
Ease the dependency on FFI

It tested well even with older version.