Add missing OPENSSL_clear_free before using ec->key

Fixes #7657
Reviewed-by: NNicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7877)

crypto/cms/cms_pwri.c

@@ -373,6 +373,7 @@ int cms_RecipientInfo_pwri_crypt(CMS_ContentInfo *cms, CMS_RecipientInfo *ri,
             goto err;
         }
 
+        OPENSSL_clear_free(ec->key, ec->keylen);
         ec->key = key;
         ec->keylen = keylen;