ALSA: hda: cs35l41: Fix NULL pointer dereference in cs35l41_get_acpi_mute_state()

Return value of a function acpi_evaluate_dsm() is dereferenced  without
checking for NULL, but it is usually checked for this function.

acpi_evaluate_dsm() may return NULL, when acpi_evaluate_object() returns
acpi_status other than ACPI_SUCCESS, so add a check to prevent the crach.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 447106e9 ("ALSA: hda: cs35l41: Support mute notifications for CS35L41 HDA")
Cc: stable@vger.kernel.org
Signed-off-by: Denis Arefev <arefev@swemel.ru>
Signed-off-by: Takashi Iwai <tiwai@suse.de>