ALSA: synth: emux: soundfont.c: divide by zero in calc_gus_envelope_time()

This function is called from load_guspatch() and the rate is specified
by the user.  If they accidentally selected zero then it would crash the
kernel.  I've just changed the zero to a one.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>