Turn 'if' condition around to avoid Svace complaint

Turn 'if' condition around to avoid Svace complaint

The unwritten assumption of this code is that both events->head and
events->tail are NULL together (an empty list) or they aren't.  So the
code was testing events->head for nullness and using that as a cue to
deference events->tail, which annoys the Svace static code analyzer.
We can silence it by testing events->tail member instead, and add an
assertion about events->head to ensure it's all consistent.

This code is very old and as far as we know, there's never been a bug
report related to this, so there's no need to backpatch.

This was found by the ALT Linux Team using Svace.

Author: Alexander Kuznetsov <kuznetsovam@altlinux.org>
Discussion: https://postgr.es/m/6d0323c3-3f5d-4137-af73-98a5ab90e77c@altlinux.org