Segfault if poll() called from delivery_callback

Description
===========

Calling poll() from the delivery_callback results in a segfault. It should raise an exception instead.

How to reproduce
================

```
from confluent_kafka import Producer

p = Producer({'bootstrap.servers': ...})

def delivery_callback(err, msg):
p.poll(0)

for d in range(5):
p.produce('test', str(d), callback=delivery_callback)

p.flush()
```

Checklist
=========
Please provide the following information:

- [x] confluent-kafka-python and librdkafka version (`confluent_kafka.version()` and `confluent_kafka.libversion()`): `('0.11.5', 722176), ('0.11.5', 722431)`
- [x] Apache Kafka broker version: 2.11-1.1.0
- [ ] Client configuration: `{...}`
- [x] Operating system: linux
- [ ] Provide client logs (with `'debug': '..'` as necessary)
- [ ] Provide broker log excerpts
- [ ] Critical issue