sd-journal: check retval of sd_id128_from_string call

Fixes: CID#1469712

CID 1469712 (#1 of 1): Unused value (UNUSED_VALUE)
returned_value: Assigning value from sd_id128_from_string(word + 2, &boot_id) to r here,
but that stored value is overwritten before it can be used.

(cherry picked from commit c9f5ac0)
(cherry picked from commit 73a327d)

Author: sshedi

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

@@ -989,6 +989,8 @@ _public_ int sd_journal_seek_cursor(sd_journal *j, const char *cursor) {
                 case 'b':
                         boot_id_set = true;
                         r = sd_id128_from_string(word + 2, &boot_id);
+                        if (r < 0)
+                                return r;
                         break;
 
                 case 'm':