Fix possible usage of NULL pointers in apps/spkac.c

Check return value of NETSCAPE_SPKI_new() and
NETSCAPE_SPKI_b64_encode(), and also clean up coding style incidentally.

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3647)
(cherry picked from commit f2582f08)