test/bdev: initialize descriptor array in open_write_test

A later patch in this series fails the test pool because
scan-build says we might be using garbage for one of the
descriptor pointer comparisons.  That later patch does not
touch this unit test file at all, and the bdev-related changes
do not touch the code path used around the bug report.  But
the report is valid - it's possible that with the additional
bdev.c changes that scan-build's ability to detect this
bug changed for the better.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I2bf165fe4d91a13d481010176a0339b591b16014

Reviewed-on: https://review.gerrithub.io/378825


Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>