Fix segmentation fault during the destruction of ConsumerImpl

[BewareMyPower]

Motivation

When I ran the tests of Python wrapper in my local env, I observed a segmentation fault. See the key stacktrace:

#3  0x00007ffff6d742c5 in std::unique_lock<std::mutex>::lock() () from /usr/local/lib/python3.8/dist-packages/_pulsar.cpython-38-x86_64-linux-gnu.so
#4  0x00007ffff6d72523 in std::unique_lock<std::mutex>::unique_lock(std::mutex&) ()
   from /usr/local/lib/python3.8/dist-packages/_pulsar.cpython-38-x86_64-linux-gnu.so
#5  0x00007ffff67de193 in pulsar::ClientImpl::newRequestId **(this=0x0)** at /home/xyz/github.com/apache/pulsar-client-cpp/lib/ClientImpl.cc:644
#6  0x00007ffff685d2c2 in pulsar::ConsumerImpl::~ConsumerImpl (this=0x7fff9800f9e0, __in_chrg=<optimized out>)
    at /home/xyz/github.com/apache/pulsar-client-cpp/lib/ConsumerImpl.cc:116

In the destructor of ConsumerImpl, client->newRequestId might be called. However, client might be a null pointer because it's returned by std::weak_ptr::lock().

Modifications

Add null check to avoid the segfault.

[BewareMyPower]

@RobertIndie PTAL first since you are the release manager of 3.1.0. I will depend on C++ client 3.1.0 on Python client. It's hard to reproduce because there is still something wrong with the Python client in my local env and I'm not sure why the current branch cannot expose this bug.