k/fetch_request: fixed iterating over partition responses

The fetch_response iterator wrapper created by operation context was
incorrectly incrementing only the partition_response iterator. This
caused it to access out of range elements when multiple topics were
present in the request.

Fixes: #180

Signed-off-by: Michal Maslanka <[email protected]>

src/v/kafka/requests/fetch_request.cc

@@ -750,7 +750,7 @@ void op_context::response_iterator::set(
 }
 
 op_context::response_iterator& op_context::response_iterator::operator++() {
-    _it->partition_response++;
+    _it++;
     return *this;
 }