time-util: encode our assumption that clock_gettime() never can return 0 or USEC_INFINITY

We generally assume that valid times returned by clock_gettime() are > 0
and < USEC_INFINITY. If this wouldn't hold all kinds of things would
break, because we couldn't distuingish our niche values from regular
values anymore.

Let's hence encode our assumptions in C, already to help static
analyzers and LLMs.

Inspired by: https://github.com/systemd/systemd/pull/41601#pullrequestreview-4094645891

src/basic/time-util.c

@@ -49,7 +49,15 @@ usec_t now(clockid_t clock_id) {
 
         assert_se(clock_gettime(map_clock_id(clock_id), &ts) == 0);
 
-        return timespec_load(&ts);
+        usec_t n = timespec_load(&ts);
+
+        /* We use both 0 and USEC_INFINITY as niche values. If the current time collides with either, things are
+         * really weird and really broken. Let's not allow this to go through, it would break too many of our
+         * assumptions in code. */
+        assert(n > 0);
+        assert(n < USEC_INFINITY);
+
+        return n;
 }
 
 nsec_t now_nsec(clockid_t clock_id) {
@@ -57,7 +65,12 @@ nsec_t now_nsec(clockid_t clock_id) {
 
         assert_se(clock_gettime(map_clock_id(clock_id), &ts) == 0);
 
-        return timespec_load_nsec(&ts);
+        nsec_t n = timespec_load_nsec(&ts);
+
+        assert(n > 0);
+        assert(n < NSEC_INFINITY);
+
+        return n;
 }
 
 dual_timestamp* dual_timestamp_now(dual_timestamp *ts) {