Fix incomplete check on CMS_SharedInfo_encode

CMS_SharedInfo_encode() can also return a negative error value, but this
is not checked in the current check, only the zero error return value is
covered. A previous PR [1] fixed the other caller's check of
CMS_SharedInfo_encode in this file, but it seems like this place was
missed. Fix it by changing the check to <= 0.

[1] a752fc4

CLA: trivial

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #20181)

crypto/cms/cms_ec.c

@@ -346,7 +346,7 @@ static int ecdh_cms_encrypt(CMS_RecipientInfo *ri)
 
     penclen = CMS_SharedInfo_encode(&penc, wrap_alg, ukm, keylen);
 
-    if (penclen == 0)
+    if (penclen <= 0)
         goto err;
 
     if (EVP_PKEY_CTX_set0_ecdh_kdf_ukm(pctx, penc, penclen) <= 0)