chardev/baum: fix baum that releases brlapi twice

Error process of baum_chr_open needs to set brlapi null, so it won't
get released twice in char_braille_finalize, which will cause
"/usr/bin/qemu-system-x86_64: double free or corruption (!prev)"

Signed-off-by: Liang Yan <lyan@suse.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
