Handle null characters in topic in BaseProducer::send (#782)

Author: esensar

src/producer/base_producer.rs

@@ -456,7 +456,10 @@ where
         }
         let (payload_ptr, payload_len) = as_bytes(record.payload);
         let (key_ptr, key_len) = as_bytes(record.key);
-        let topic_cstring = CString::new(record.topic).unwrap();
+        let topic_cstring = match CString::new(record.topic) {
+            Ok(topic) => topic,
+            Err(nul_error) => return Err((nul_error.into(), record)),
+        };
         let opaque_ptr = record.delivery_opaque.into_ptr();
         let produce_error = unsafe {
             rdsys::rd_kafka_producev(