journald: properly update message size after stripping the identifier

Valgrind says:
==29176== Conditional jump or move depends on uninitialised value(s)
==29176==    at 0x412A85: cunescape_length_with_prefix (util.c:1565)
==29176==    by 0x40B351: dev_kmsg_record (journald-kmsg.c:301)
==29176==    by 0x40B653: server_read_dev_kmsg (journald-kmsg.c:347)
==29176==    by 0x40B701: server_flush_dev_kmsg (journald-kmsg.c:365)
==29176==    by 0x409DE7: main (journald.c:1535)