Fix error-path memory leak in asn_mime.c

Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5142)

crypto/asn1/asn_mime.c

@@ -425,6 +425,7 @@ ASN1_VALUE *SMIME_read_ASN1(BIO *bio, BIO **bcont, const ASN1_ITEM *it)
             || hdr->value == NULL) {
             sk_MIME_HEADER_pop_free(headers, mime_hdr_free);
             ASN1err(ASN1_F_SMIME_READ_ASN1, ASN1_R_NO_SIG_CONTENT_TYPE);
+            sk_BIO_pop_free(parts, BIO_vfree);
             return NULL;
         }