in do_alt_speller(), if we can't invoke the spell checker, use sprintf()

in do_alt_speller(), if we can't invoke the spell checker, use sprintf()
instead of snprintf() to write the error string we return, as
altspell_error will always be long enough to hold it

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2813 35c25a1d-7b9e-4130-9fde-d3aeb78583b8