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 c9f5ac0917409cd9eb3d55b72c2443d9b5374709)