null: fix compile time warning on OpenBSD

null: fix compile time warning on OpenBSD

Fix the following warning on OpenBSD 6.2 (gcc 4.2.1).
This can probably be silenced with void** argument and memcpy() too,
but returning a non-void pointer is clearer IMO when the code also
targets C++.

Compiles with regular make/gmake as well as
 # g++ -O2 -g -shared -rdynamic -fPIC -o cpp_null null.c -DFIO_EXTERNAL_ENGINE

--
    CC engines/null.o
engines/null.c: In function 'fio_null_init':
engines/null.c:142: warning: dereferencing type-punned pointer will break strict-aliasing rules

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>