test/kv_store_bench.cc: fix coverity REVERSE_INULL

CID 1021214 (#1 of 1): Dereference before null check (REVERSE_INULL)
 check_after_deref: Null-checking cb_args suggests that it may be
 null, but it has already been dereferenced on all paths leading to
 the check.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>