unit_tests: initialize a field to silence a valgrind report

A memory chunk is split in two, and the concatenation of the
two parts compared to the original, triggering a comparison
with uninitialized (with a copy of itself, but valgrind does
not realize that).

tests/unit_tests/epee_levin_protocol_handler_async.cpp

@@ -242,6 +242,7 @@ namespace
       m_req_head.m_cb = m_in_data.size();
       m_req_head.m_have_to_return_data = true;
       m_req_head.m_command = expected_command;
+      m_req_head.m_return_code = 0;
       m_req_head.m_flags = LEVIN_PACKET_REQUEST;
       m_req_head.m_protocol_version = LEVIN_PROTOCOL_VER_1;