libsepol: fix policydb_read for policy versions < 24

Policy versions < 24 did not include type attributes in the types symtab,
so there can be holes in the type_val_to_struct array.  Fixes a segfault
during the downgrade test performed by a make test in libsepol.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
1 file changed