iio: dummy: iio_simple_dummy: check the return value of kstrdup()

iio: dummy: iio_simple_dummy: check the return value of kstrdup()

kstrdup() is also a memory allocation-related function, it returns NULL
when some memory errors happen. So it is better to check the return
value of it so to catch the memory error in time. Besides, there should
have a kfree() to clear up the allocation if we get a failure later in
this function to prevent memory leak.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Link: https://lore.kernel.org/r/tencent_C920CFCC33B9CC1C63141FE1334A39FF8508@qq.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>