[ALSA] Fix potential NULL pointer dereference in echoaudio midi

Fix races between the timer handler and the close function.

Signed-off-by: Giuliano Pochini <pochini@shiny.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>