Continue standardisation of malloc handling in apps

continue on from previous commits but in the apps directory

Reviewed-by: Kurt Roeckx <kurt@openssl.org>

apps/spkac.c

diff --git a/apps/spkac.c b/apps/spkac.c
index eaeb3c1..9cbe7fe 100644
--- a/apps/spkac.c
+++ b/apps/spkac.c
@@ -189,7 +189,7 @@
 
     spkstr = NCONF_get_string(conf, spksect, spkac);
 
-    if (!spkstr) {
+    if (spkstr == NULL) {
         BIO_printf(bio_err, "Can't find SPKAC called \"%s\"\n", spkac);
         ERR_print_errors(bio_err);
         goto end;