unit: when JobTimeoutSec= is turned off, implicitly turn off JobRunningTimeoutSec= too

We added JobRunningTimeoutSec= late, and Dracut configured only
JobTimeoutSec= to turn of root device timeouts before. With this change
we'll propagate a reset of JobTimeoutSec= into JobRunningTimeoutSec=,
but only if the latter wasn't set explicitly.

This should restore compatibility with older systemd versions.

Fixes: #6402

src/basic/time-util.c

@@ -1082,7 +1082,11 @@ int parse_sec(const char *t, usec_t *usec) {
 }
 
 int parse_sec_fix_0(const char *t, usec_t *usec) {
+        assert(t);
+        assert(usec);
+
         t += strspn(t, WHITESPACE);
+
         if (streq(t, "0")) {
                 *usec = USEC_INFINITY;
                 return 0;