http: Avoid crash when g_thread_http was never started

g_thread_http can not be joined when it is not joinable. Avoid crashing
the node by adding the required check and add a test.

test/functional/rpc_users.py

@@ -99,5 +99,13 @@ class HTTPBasicsTest(BitcoinTestFramework):
 
         self.test_auth(self.nodes[1], self.rpcuser, self.rpcpassword)
 
+        self.log.info('Check that failure to write cookie file will abort the node gracefully')
+        self.stop_node(0)
+        cookie_file = os.path.join(get_datadir_path(self.options.tmpdir, 0), self.chain, '.cookie.tmp')
+        os.mkdir(cookie_file)
+        init_error = 'Error: Unable to start HTTP server. See debug log for details.'
+        self.nodes[0].assert_start_raises_init_error(expected_msg=init_error)
+
+
 if __name__ == '__main__':
     HTTPBasicsTest().main()