AioTestData: initialize m_ioctx with NULL

CID 717392 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "m_ioctx" is not initialized
 in this constructor nor in any functions that it calls.

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