tests: mem_protect: ensure allocated objects are initialized

K_OBJ_MSGQ, K_OBJ_PIPE, and K_OBJ_STACK objects have pointers
to additional memory that can be allocated. The k_obj_alloc()
returns these objects as uninitialized so when they are freed
there are random opportunities for freeing invalid memory
and causing random faults.

Signed-off-by: David Leach <david.leach@nxp.com>
(cherry picked from commit fdea2a62)