journal: add object sanity check to journal_file_move_to_object()

Introduce journal_file_check_object(), which does lightweight object
sanity checks, and use it in journal_file_move_to_object(), so that we
will catch certain corrupted objects in the journal file.

This fixes #6447, where we had only partially written out OBJECT_ENTRY
(ObjectHeader written, but rest of object zero bytes), causing
"journalctl --list-boots" to fail.

  $ builddir.vanilla/journalctl --list-boots -D bug6447/
  Failed to determine boots: No data available

  $ builddir.patched/journalctl --list-boots -D bug6447/
  -52 22633da1c5374a728d6c215e2c301dc2 Mon 2017-07-10 05:29:21 EEST—Mon 2017-07-10 05:31:51 EEST
  -51 2253aab9ea7e4a2598f2abda82939eff Mon 2017-07-10 05:32:22 EEST—Mon 2017-07-10 05:36:49 EEST
  -50 ef0d85d35c74486fa4104f9d6391b6ba Mon 2017-07-10 05:40:33 EEST—Mon 2017-07-10 05:40:40 EEST
  [...]

Note that journal_file_check_object() is similar to
journal_file_object_verify(). The most expensive checks are omitted, as
they would slow down every journal_file_move_to_object() call too much.

With this implementation, the added overhead is small, for example when
dumping some journal content to /dev/null
(built with -Dbuildtype=debugoptimized -Db_ndebug=true):

 Performance counter stats for 'builddir.vanilla/journalctl -D 76f4d4c3406945f9a60d3ca8763aa754/':

      12542,311634      task-clock:u (msec)       #    1,000 CPUs utilized
                 0      context-switches:u        #    0,000 K/sec
                 0      cpu-migrations:u          #    0,000 K/sec
            80 100      page-faults:u             #    0,006 M/sec
    41 786 963 456      cycles:u                  #    3,332 GHz
   105 453 864 770      instructions:u            #    2,52  insn per cycle
    24 342 227 334      branches:u                # 1940,809 M/sec
       105 709 217      branch-misses:u           #    0,43% of all branches

      12,545199291 seconds time elapsed

 Performance counter stats for 'builddir.patched/journalctl -D 76f4d4c3406945f9a60d3ca8763aa754/':

      12734,723233      task-clock:u (msec)       #    1,000 CPUs utilized
                 0      context-switches:u        #    0,000 K/sec
                 0      cpu-migrations:u          #    0,000 K/sec
            80 693      page-faults:u             #    0,006 M/sec
    42 661 017 429      cycles:u                  #    3,350 GHz
   107 696 985 865      instructions:u            #    2,52  insn per cycle
    24 950 526 745      branches:u                # 1959,252 M/sec
       101 762 806      branch-misses:u           #    0,41% of all branches

      12,737527327 seconds time elapsed

Fixes #6447.