Standardise our style for checking malloc failures

if we have a malloc |x = OPENSSL_malloc(...)| sometimes we check |x|
for NULL and sometimes we treat it as a boolean |if(!x) ...|. Standardise
the approach in libssl.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
diff --git a/ssl/ssl_conf.c b/ssl/ssl_conf.c
index 9c252fa..ad20f44 100644
--- a/ssl/ssl_conf.c
+++ b/ssl/ssl_conf.c
@@ -487,12 +487,12 @@
     BIO *in = NULL;
     if (cctx->ctx || cctx->ssl) {
         in = BIO_new(BIO_s_file());
-        if (!in)
+        if (in == NULL)
             goto end;
         if (BIO_read_filename(in, value) <= 0)
             goto end;
         dh = PEM_read_bio_DHparams(in, NULL, NULL, NULL);
-        if (!dh)
+        if (dh == NULL)
             goto end;
     } else
         return 1;