ALSA: synth: missing check for possible NULL after the call to kstrdup

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@raspberrypi


Signed-off-by: Takashi Iwai <tiwai@suse.de>