vdpa/mlx5: fix memory allocation failure checks

The memory allocation failure checking for in and out is currently
checking if the pointers are valid rather than the contents of what
they point to. Hence the null check on failed memory allocations is
incorrect.  Fix this by adding the missing indirection in the check.
Also for the default case, just set the *in and *out to null as
these don't have any thing allocated to kfree. Finally remove the
redundant *in and *out check as these have been already done on each
allocation in the case statement.

Addresses-Coverity: ("Null pointer dereference")
Fixes: 1a86b377

 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200806160828.90463-1-colin.king@canonical.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Eli Cohen <eli@mellanox.com>