gateway: wrong topic id returned in PUBACK

[aurel32]

Since commit f079211 the gateway returns PUBACK messages with the wrong topic id in response to PUBLISH messages. For example:

PUBLISH message:

00000000  1d 0c 20 00 04 09 66 7b  22 74 65 6d 70 65 72 61  |.. ...f{"tempera|
00000010  74 75 72 65 22 3a 20 32  37 2e 32 35 7d           |ture": 27.25}|

PUBACK message:

07 0d 15 a0 09 66 00                              |.....f.|

Note how the message ID 2406 matches, while the topic ID do not (4 vs 5536).