ALSA: seq: Clear padded bytes at expanding events

There can be a small memory hole that may not be cleared at expanding
an event with the variable length type.  Make sure to clear it.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-18-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>