commit be3bb823 upstream.

There was no check about the id string of user control elements, so we
accepted even a control element with an empty string, which is
obviously bogus.  This patch adds more sanity checks of id strings.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>