sd-journal: verify monotonic timestamp before assigning result

Previously, ret_boot_id was assigned even when the function failed due
to an invalid monotonic timestamp stored for a journal entry.

(cherry picked from commit c9df471)

src/libsystemd/sd-journal/sd-journal.c

@@ -2526,9 +2526,7 @@ _public_ int sd_journal_get_monotonic_usec(sd_journal *j, uint64_t *ret, sd_id12
         if (r < 0)
                 return r;
 
-        if (ret_boot_id)
-                *ret_boot_id = o->entry.boot_id;
-        else {
+        if (!ret_boot_id) {
                 sd_id128_t id;
 
                 r = sd_id128_get_boot(&id);
@@ -2545,6 +2543,8 @@ _public_ int sd_journal_get_monotonic_usec(sd_journal *j, uint64_t *ret, sd_id12
 
         if (ret)
                 *ret = t;
+        if (ret_boot_id)
+                *ret_boot_id = o->entry.boot_id;
 
         return 0;
 }