queue: Fix STAILQ_SWAP

The debug assertions added in commit 34740937f7a4 do not hold for a
window in STAILQ_SWAP, so check whether the queues are empty directly.

Reported by: ler
Fixes: 34740937f7a4 ("queue: New debug macros for STAILQ")

(cherry picked from commit da2c88dfcf4f425e6e0a58d6df3a7c8e88d8df92)