Fix crash when new topic is not created.

[elektito]

I found that the following code caused a crash:

from confluent_kafka.admin import AdminClient
from confluent_kafka.cimpl import NewTopic

client = AdminClient({})
topics = [NewTopic(topic='foo', num_partitions=0)]
client.create_topics(topics)

This pull request, fixes the issue. The problem was that when a topic failed to be created, like in this case where the number of partitions is zero, we would still try to free the topic, causing a null pointer to be de-referenced in rdkafka.

While, this is also a bug in rdkafka, since a null pointer should not be de-referenced either way, we should not pass such a pointer to a library either.

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[elektito]

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[edenhill]

Good fix, thank you!