If kcalloc() return NULL due to memory starvation, it is possible for
kstrdup() to return NULL in similar case. So add null check after the call
to kstrdup() is made.

[ minor coding-style fix by tiwai ]
Signed-off-by: Austin Kim <austin.kim@lge.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20211109003742.GA5423@raspberrypiSigned-off-by: Takashi Iwai <tiwai@suse.de>