CMS_get0_SignerInfos(): Prevent spurious error on cms_get0_signed() failure

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12959)

crypto/cms/cms_sd.c

@@ -496,9 +496,13 @@ void ossl_cms_SignerInfos_set_cmsctx(CMS_ContentInfo *cms)
 {
     int i;
     CMS_SignerInfo *si;
-    STACK_OF(CMS_SignerInfo) *sinfos = CMS_get0_SignerInfos(cms);
+    STACK_OF(CMS_SignerInfo) *sinfos;
     const CMS_CTX *ctx = ossl_cms_get0_cmsctx(cms);
 
+    ERR_set_mark();
+    sinfos = CMS_get0_SignerInfos(cms);
+    ERR_pop_to_mark(); /* removes error in case sinfos == NULL */
+
     for (i = 0; i < sk_CMS_SignerInfo_num(sinfos); i++) {
         si = sk_CMS_SignerInfo_value(sinfos, i);
         if (si != NULL)