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

commit 6102c48bd421074a33e102f2ebda3724e8d275f9 upstream.

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

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