LU-9307 lov: NULL pointer deref in lov_delete_composite

LU-9307 lov: NULL pointer deref in lov_delete_composite

When lov_init_composite() cannot allocate memory and bails out,
lov_delete_composite() is then called, and it should check the
composite entry's existence before trying to delete each component.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I013c596a19a114b763f3eebbaafa1559fa09d8d9
Reviewed-on: https://review.whamcloud.com/26458
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>