alsaucm: Don't double-free empty lists

alsaucm: Don't double-free empty lists

When snd_use_case_get_list (and hence also snd_use_case_card_list) returns
an empty list, alsaucm still attempts to free it. This ends up double-
freeing the returned list, or worse, freeing an invalid pointer, depending
on how snd_use_case_get_list gets implemented. Fix alsaucm to return early
on empty lists to avoid this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>