libsepol/cil: cil_reset_classperms_set() should not reset classpermission

In struct cil_classperms_set, the set field is a pointer to a
struct cil_classpermission which is looked up in the symbol table.
Since the cil_classperms_set does not create the cil_classpermission,
it should not reset it.

Set the set field to NULL instead of resetting the classpermission
that it points to.

Signed-off-by: James Carter <jwcart2@gmail.com>