staging: speakup: ensure we do not overrun synths array

synth_add allows one to add MAXSYNTHS synths to the synths array;
however it always NULLifies the next synth in the array which
means that on the MAXSYNTHS synth we get an out-of-bounds write of
the NULL to the synths array.  Make the synths array MAXSYNTHS + 1
elements in size to allow for the final NULL sentinal to avoid the
out-of-bounds write.

Issue found wit CoverityScan, CID#744671

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>