staging:iio:impedance:ad5933: correct error check

iio_kfifo_allocate returns NULL in case of error.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression *x;
identifier f;
statement S1,S2;
@@

*x = f(...);
 if (x) { <+... when != if (...) S1 else S2
     -ENOMEM ...+> }
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
cc: stable@vger.kernel.org