Use p==NULL not !p (in if statements, mainly)

Reviewed-by: Tim Hudson <tjh@openssl.org>

apps/req.c

@@ -962,7 +962,7 @@ static int build_subject(X509_REQ *req, char *subject, unsigned long chtype,
 {
     X509_NAME *n;
 
-    if (!(n = parse_name(subject, chtype, multirdn)))
+    if ((n = parse_name(subject, chtype, multirdn)) == NULL)
         return 0;
 
     if (!X509_REQ_set_subject_name(req, n)) {