radius: Fix list traversal in rc_avpair_insert

radius: Fix list traversal in rc_avpair_insert

In rc_avpair_insert, if the list element "p" is non-NULL but not
actually in the list "a", we can end up with this_node being NULL and
being dereferenced.

By changing the while test to this_node->next we avoid having
this_node being NULL; the loop will terminate when this_node == p or
this_node->next == NULL, which is what we want.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>