libsemanage: Fix null pointer dereference in semanage_module_key_destroy

If modkey is NULL, semanage_module_key_destroy() would still try to
initialize a modkey after freeing it.

Signed-off-by: Yuli Khodorkovskiy <ykhodorkovskiy@tresys.com>
Signed-off-by: James Carter <jwcart2@tycho.nsa.gov>

libsemanage/src/modules.c

@@ -690,10 +690,12 @@ int semanage_module_key_destroy(semanage_handle_t *sh,
 {
  assert(sh);
 
- if (modkey) {
- free(modkey->name);
+ if (!modkey) {
+ return 0;
  }
 
+ free(modkey->name);
+
  return semanage_module_key_init(sh, modkey);
 }