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.

(cherry picked from commit e84914fd)

tests/online/clone.c

@@ -263,6 +263,9 @@ 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");
 
@@ -293,6 +296,9 @@ 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");