iio: multiplexer: add NULL check on devm_kzalloc() and devm_kmemdup() return values

Check return values from call to devm_kzalloc() and devm_kmemup()
in order to prevent a NULL pointer dereference.

This issue was detected using Coccinelle and the following semantic patch:

@@
expression x;
identifier fld;
@@

* x = devm_kzalloc(...);
   ... when != x == NULL
   x->fld

Fixes: 7ba9df54 ("iio: multiplexer: new iio category and iio-mux driver")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>