staging: speakup: avoid out-of-range access in synth_init()

commit ae428655b826f2755a8101b27beda42a275ef8ad upstream.

Check that array index is in-bounds before accessing the synths[] array.

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>