Fix detected by PVS-Studio (mfontanini#167)

* Fix PVS-Studio
V591 	Non-void function should return a value.
V519 	The 'new_item' variable is assigned values twice successively.

Author: pavel-pimenov

include/cppkafka/header_list_iterator.h

@@ -78,6 +78,7 @@ class HeaderIterator {
         header_list_ = other.header_list_;
         header_ = make_header(other.header_);
         index_ = other.index_;
+        return *this;
     }
     HeaderIterator(HeaderIterator&&) = default;
     HeaderIterator& operator=(HeaderIterator&&) = default;

src/topic_partition_list.cpp

@@ -45,10 +45,11 @@ TopicPartitionsListPtr convert(const TopicPartitionList& topic_partitions) {
     TopicPartitionsListPtr handle(rd_kafka_topic_partition_list_new(topic_partitions.size()),
                                   &rd_kafka_topic_partition_list_destroy);
     for (const auto& item : topic_partitions) {
-        rd_kafka_topic_partition_t* new_item = nullptr;
-        new_item = rd_kafka_topic_partition_list_add(handle.get(),
-                                                     item.get_topic().data(),
-                                                     item.get_partition());
+        rd_kafka_topic_partition_t* new_item = rd_kafka_topic_partition_list_add(
+            handle.get(),
+            item.get_topic().data(),
+            item.get_partition()
+        );
         new_item->offset = item.get_offset();
     }
     return handle;