ucm: fix crash when calling snd_use_case_geti() with no device or modifier

ucm: fix crash when calling snd_use_case_geti() with no device or modifier

When calling snd_use_case_geti(uc_mgr, "_devstatus", &lvalue) the code
ends up calling device_status(uc_mgr, NULL), which result in a crash in
strcmp(dev->name, NULL), when there are enabled devices.

This happens because snd_use_case_geti() allows a "_devstatus"
identifier even if it's only supposed to allow the form
"_devstatus/{device}".

So check that the device name is not null.

The same issue occurs with "_modstatus", this change fixes that as well.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>