ctl/rawmidi/hwdep: fix compiler warning due to uninitialized variables

ctl/rawmidi/hwdep: fix compiler warning due to uninitialized variables

In some libraries, the local variable of udev enumerate is carelessly
uninitialized. As a result, compiler generates warnings, like:

../src/ctl/query.c: In function ‘alsactl_get_card_id_list’:
../src/ctl/query.c:131:8: warning: ‘enumerator’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  131 |     if (enumerator == NULL)
      |        ^

This commit fixes the bug.

Fixes: d1936fbbe7e2 ("ctl: add global method to get list of available sound cards")
Fixes: cb128c5b59e0 ("rawmidi; add global method to get list of rawmidi device for sound card")
Fixes: d4a9f6735d05 ("hwdep: add global method to get list of hwdep devices in sound card")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>