Prevent crash on missing signedAttrs

Though RPKI enforces the presence of this field, it is very much
optional in CMS.
Also adds missing validation messages in relevant error paths.

Thanks to Niklas Vogel for reporting this.

src/asn1/signed_data.c

@@ -468,30 +468,32 @@ get_content_type_attr(struct SignedData *sdata, OBJECT_IDENTIFIER_t **result)
 	bool equal;
 
 	if (sdata == NULL)
-		return -EINVAL;
+		return pr_val_err("SignedData is NULL.");
 	if (sdata->signerInfos.list.array == NULL)
-		return -EINVAL;
+		return pr_val_err("SignerInfos array is NULL.");
 	if (sdata->signerInfos.list.array[0] == NULL)
-		return -EINVAL;
+		return pr_val_err("SignerInfos array first element is NULL.");
 
 	signedAttrs = sdata->signerInfos.list.array[0]->signedAttrs;
+	if (signedAttrs == NULL)
+		return pr_val_err("signedAttrs is NULL.");
 	if (signedAttrs->list.array == NULL)
-		return -EINVAL;
+		return pr_val_err("signedAttrs array is NULL.");
 
 	for (i = 0; i < signedAttrs->list.count; i++) {
 		attr = signedAttrs->list.array[i];
 		if (!attr)
-			return -EINVAL;
+			return pr_val_err("signedAttrs array element %d is NULL.", i);
 		error = oid2arcs(&attr->attrType, &arcs);
 		if (error)
-			return -EINVAL;
+			return error;
 		equal = ARCS_EQUAL_OIDS(&arcs, oid_cta);
 		free_arcs(&arcs);
 		if (equal) {
 			if (attr->attrValues.list.array == NULL)
-				return -EINVAL;
+				return pr_val_err("signedAttrs attrValue array is NULL.");
 			if (attr->attrValues.list.array[0] == NULL)
-				return -EINVAL;
+				return pr_val_err("signedAttrs attrValue array first element is NULL.");
 			return asn1_decode_any(attr->attrValues.list.array[0],
 			    &asn_DEF_OBJECT_IDENTIFIER,
 			    (void **) result, true);