LU-14389 lov: avoid NULL dereference in cleanup

LU-14389 lov: avoid NULL dereference in cleanup

Running racer concurrently with file migration crashes easily
when the layout changes for a file in an unexpected way:

  lov_init_composite() lustre-clilov: DOM entries with different sizes
  lov_layout_change() lustre-clilov: cannot apply new layout on
      [0x200000402:0x3e6a:0x0] : rc = -22
  BUG: unable to handle kernel NULL pointer dereference at 0x00000014
  IP: [<ffffffffa08baef4>] lov_delete_composite+0x104/0x540 [lov]
  Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
  CPU: 1 PID: 20227 Comm: ln

Avoid the NULL dereference if the entry is not fully initialized
during cleanup.

Test-Parameters: testlist=racer env=DURATION=3600
Fixes: 61a002cd863 ("LU-13602 flr: skip unknown FLR component types")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I8fe17f1b49ca2bccc7a285febe47032d023ebbe5
Reviewed-on: https://review.whamcloud.com/41398
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>