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

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

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