Fix remaining static analysis warnings

Fix or suppress the remaining warnings generated by static analysis.
This is a necessary step before we can incorporate static analysis into
CI. Where possible, I've preferred to modify the code itself instead of
just disabling the warning with a magic comment, so that if we decide to
use different static analysis tools in the future we will be covered
against them raising similar warnings.

src/decay.c

@@ -157,6 +157,7 @@ decay_deadline_reached(const decay_t *decay, const nstime_t *time) {
 uint64_t
 decay_npages_purge_in(decay_t *decay, nstime_t *time, size_t npages_new) {
 	uint64_t decay_interval_ns = decay_epoch_duration_ns(decay);
+	assert(decay_interval_ns != 0);
 	size_t n_epoch = (size_t)(nstime_ns(time) / decay_interval_ns);
 
 	uint64_t npages_purge;