Fix unit test from asserting

7d9a75df uncovered that slurmdb_pack_assoc_usage() expects a usage
ptr.

testsuite/slurm_unit/common/slurmdb_pack/pack_assoc_rec_with_usage-test.c

@@ -15,6 +15,8 @@ START_TEST(invalid_protocol)
 	uint32_t x;
 
 	slurmdb_assoc_rec_t *assoc_rec = xmalloc(sizeof(slurmdb_assoc_rec_t));
+	assoc_rec->usage = xmalloc(sizeof(slurmdb_assoc_usage_t));
+
 	Buf buf = init_buf(1024);
 
 	pack32(22, buf);