Domain name resolution failed causes core dumped

## Description
When there are unresolvable domain names, there will be occasional core dumped.
## env
rocketmq-4.9.2
rocketmq-client-python-1.2.0
rocketmq-client-cpp-2.1.0

## error info
```bash
[root@localhost tmp]# /opt/env/miniconda3/bin/python3 test_mq.py
start consume message
Traceback (most recent call last):
File "test_mq.py", line 20, in
start_consume_message()
File "test_mq.py", line 13, in start_consume_message
consumer.start()
File "/opt/env/miniconda3/lib/python3.7/site-packages/rocketmq/client.py", line 370, in start
ffi_check(dll.StartPushConsumer(self._handle))
File "/opt/env/miniconda3/lib/python3.7/site-packages/rocketmq/exceptions.py", line 44, in ffi_check
raise exc_cls(msg)
rocketmq.exceptions.PushConsumerStartFailed: Failed to resolve host name(testmq1.mq): nodename nor servname provided, or not known,error:-1,in file line:103
terminate called after throwing an instance of 'rocketmq::MQClientException'
what(): msg: Failed to resolve host name(testmq1.mq): nodename nor servname provided, or not known,error:-1,in file line:103
Aborted (core dumped)
```
## code
```python
from rocketmq.client import PushConsumer, ConsumeStatus
import time

def callback(msg):
print(msg.id, msg.body, msg.get_property('property'))
return ConsumeStatus.CONSUME_SUCCESS

def start_consume_message():
consumer = PushConsumer('consumer_group')
consumer.set_name_server_address('testmq1.mq:9876')
consumer.subscribe('TopicTest', callback)
print ('start consume message')
consumer.start()

while True:
time.sleep(3600)

if __name__ == '__main__':
start_consume_message()
```

## gdb info
```gdb
(gdb) bt
#0 0x00007f45b42086eb in raise () from /lib64/libc.so.6
#1 0x00007f45b4209a11 in abort () from /lib64/libc.so.6
#2 0x00007f45a676e84a in __gnu_cxx::__verbose_terminate_handler ()
at /home/nwani/m3/conda-bld/compilers_linux-64_1560109574129/work/.build/x86_64-conda_cos6-linux-gnu/src/gcc/libstdc++-v3/libsupc++/vterminate.cc:95
#3 0x00007f45a676cf47 in __cxxabiv1::__terminate (handler=)
at /home/nwani/m3/conda-bld/compilers_linux-64_1560109574129/work/.build/x86_64-conda_cos6-linux-gnu/src/gcc/libstdc++-v3/libsupc++/eh_terminate.cc:47
#4 0x00007f45a676cf7d in std::terminate () at /home/nwani/m3/conda-bld/compilers_linux-64_1560109574129/work/.build/x86_64-conda_cos6-linux-gnu/src/gcc/libstdc++-v3/libsupc++/eh_terminate.cc:57
#5 0x00007f45a676d15a in __cxxabiv1::__cxa_throw (obj=0x7f457c0044a0, tinfo=0x7f45a6b6fdd8 ,
dest=0x7f45a696e1f0 )
at /home/nwani/m3/conda-bld/compilers_linux-64_1560109574129/work/.build/x86_64-conda_cos6-linux-gnu/src/gcc/libstdc++-v3/libsupc++/eh_throw.cc:95
#6 0x00007f45a6a3e817 in rocketmq::TcpTransport::getInetAddr(std::__cxx11::basic_string, std::allocator >&) () from /opt/env/miniconda3/lib/librocketmq.so
#7 0x00007f45a6a3f4c3 in rocketmq::TcpTransport::connect(std::__cxx11::basic_string, std::allocator > const&, int) () from /opt/env/miniconda3/lib/librocketmq.so
#8 0x00007f45a6a38f5d in rocketmq::TcpRemotingClient::CreateTransport(std::__cxx11::basic_string, std::allocator > const&, bool) ()
from /opt/env/miniconda3/lib/librocketmq.so
#9 0x00007f45a6a39399 in rocketmq::TcpRemotingClient::CreateNameServerTransport(bool) () from /opt/env/miniconda3/lib/librocketmq.so
#10 0x00007f45a6a3959b in rocketmq::TcpRemotingClient::GetTransport(std::__cxx11::basic_string, std::allocator > const&, bool) ()
from /opt/env/miniconda3/lib/librocketmq.so
#11 0x00007f45a6a3b764 in rocketmq::TcpRemotingClient::invokeSync(std::__cxx11::basic_string, std::allocator > const&, rocketmq::RemotingCommand&, int) ()
from /opt/env/miniconda3/lib/librocketmq.so
#12 0x00007f45a696cf8b in rocketmq::MQClientAPIImpl::getTopicRouteInfoFromNameServer(std::__cxx11::basic_string, std::allocator > const&, int, rocketmq::SessionCredentials const&) () from /opt/env/miniconda3/lib/librocketmq.so
#13 0x00007f45a697bcb5 in rocketmq::MQClientFactory::updateTopicRouteInfoFromNameServer(std::__cxx11::basic_string, std::allocator > const&, rocketmq::SessionCredentials const&, bool) () from /opt/env/miniconda3/lib/librocketmq.so
#14 0x00007f45a697f3f9 in rocketmq::MQClientFactory::updateTopicRouteInfo(boost::system::error_code&, boost::shared_ptr, boost::asio::deadline_timer_service > > >) () from /opt/env/miniconda3/lib/librocketmq.so
#15 0x00007f45a6980ba9 in boost::asio::detail::wait_handler, boost::asio::deadline_timer_service > > > >, boost::_bi::list3, boost::_bi::value, boost::_bi::value, boost::asio::deadline_timer_service > > > > > > >::do_complete(boost::asio::detail::task_io_service*, boost::asio::detail::task_io_service_operation*, boost::system::error_code const&, unsigned long) () from /opt/env/miniconda3/lib/librocketmq.so
#16 0x00007f45a6985b5a in boost::asio::detail::task_io_service::run(boost::system::error_code&) () from /opt/env/miniconda3/lib/librocketmq.so
#17 0x00007f45a697eccc in rocketmq::MQClientFactory::startScheduledTask(bool) () from /opt/env/miniconda3/lib/librocketmq.so
#18 0x00007f45a6a84f5e in thread_proxy () from /opt/env/miniconda3/lib/librocketmq.so
#19 0x00007f45b4398fed in ?? () from /lib64/libpthread.so.0
#20 0x00007f45b42cc18f in clone () from /lib64/libc.so.6
```
How to avoid this core dumped？

Hope this can be fixed, thank you very much！

@messense @ifplusor