[ALSA] seq_midi_event: fix parsing of F9/FD bytes

Check for a valid event type when encoding a system real-time message to
prevent the bytes F9 or FD resulting in an empty sequencer message.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>