ALSA: hda/ca0132 - fix possible NULL pointer use

gcc-7 caught what it considers a NULL pointer dereference:

sound/pci/hda/patch_ca0132.c: In function 'dspio_scp.constprop':
sound/pci/hda/patch_ca0132.c:1487:4: error: argument 1 null where non-null expected [-Werror=nonnull]

This is plausible from looking at the function, as we compare 'reply'
to NULL earlier in it. I have not tried to analyze if there are constraints
that make it impossible to hit the bug, but adding another NULL check in
the end kills the warning and makes the function more robust.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>