ALSA: sound/isa/gus: check the return value of kstrdup()

kstrdup() returns NULL when some internal memory errors happen, it is
better to check the return value of it. Otherwise, we may not to be able
to catch some memory errors in time.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Link: https://lore.kernel.org/r/tencent_1E3950293AC22395ACFE99404C985D738309@qq.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>