package/raptor: fix CVE-2020-25713

A malformed input file can lead to a segfault due to an out of bounds
array access in raptor_xml_writer_start_element_common.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>