(hx509_lock_free): allow freeing NULL

lib/hx509/lock.c

@@ -214,11 +214,13 @@ hx509_lock_prompt(hx509_lock lock, hx509_prompt *prompt)
 void
 hx509_lock_free(hx509_lock lock)
 {
+    if (lock) {
 	hx509_certs_free(&lock->certs);
 	hx509_lock_reset_passwords(lock);
 	memset(lock, 0, sizeof(*lock));
 	free(lock);
     }
+}
 
 int
 hx509_prompt_hidden(hx509_prompt_type type)