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

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

commit d159037abbe3412285c271bdfb9cdf19e62678ff upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>