add OSSL_STACK_OF_X509_free() for commonly used pattern

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17307)
diff --git a/ssl/ssl_rsa.c b/ssl/ssl_rsa.c
index 5509389..bba0051 100644
--- a/ssl/ssl_rsa.c
+++ b/ssl/ssl_rsa.c
@@ -955,7 +955,7 @@
         }
     }
 
-    sk_X509_pop_free(c->pkeys[i].chain, X509_free);
+    OSSL_STACK_OF_X509_free(c->pkeys[i].chain);
     c->pkeys[i].chain = dup_chain;
 
     X509_free(c->pkeys[i].x509);