unitest helper: Fix copy of packet data.

The copy of packet data was causing a memory corruption causing
weird crash.

src/util-unittest-helper.c

@@ -228,7 +228,7 @@ Packet *UTHBuildPacketReal(uint8_t *payload, uint16_t payload_len,
         /* TODO: Add more protocols */
     }
 
-    memcpy(GET_PKT_DATA(p)+hdr_offset, payload, payload_len);
+    PacketCopyDataOffset(p, hdr_offset, payload, payload_len);
     SET_PKT_LEN(p, hdr_offset + payload_len);
     p->payload = GET_PKT_DATA(p)+hdr_offset;