online::clone: free url and username before resetting

Before resetting the url and username, ensure that we free them in case
they were set by environment variables.

tests/online/clone.c

@@ -276,6 +276,9 @@ static int cred_failure_cb(
 
 void test_online_clone__cred_callback_failure_return_code_is_tunnelled(void)
 {
+	git__free(_remote_url);
+	git__free(_remote_user);
+
 	_remote_url = git__strdup("https://github.com/libgit2/non-existent");
 	_remote_user = git__strdup("libgit2test");
 
@@ -306,6 +309,9 @@ void test_online_clone__cred_callback_called_again_on_auth_failure(void)
 {
 	size_t counter = 0;
 
+	git__free(_remote_url);
+	git__free(_remote_user);
+
 	_remote_url = git__strdup("https://github.com/libgit2/non-existent");
 	_remote_user = git__strdup("libgit2test");