x509store: refactor X509::StoreContext#chain

Use ossl_x509_sk2ary() to create an array of OpenSSL::X509::Certificate
from STACK_OF(X509).

ext/openssl/ossl_x509store.c

@@ -574,26 +574,13 @@ static VALUE
 ossl_x509stctx_get_chain(VALUE self)
 {
     X509_STORE_CTX *ctx;
-    STACK_OF(X509) *chain;
-    X509 *x509;
-    int i, num;
-    VALUE ary;
+    const STACK_OF(X509) *chain;
 
     GetX509StCtx(self, ctx);
-    if((chain = X509_STORE_CTX_get0_chain(ctx)) == NULL){
-        return Qnil;
-    }
-    if((num = sk_X509_num(chain)) < 0){
-	OSSL_Debug("certs in chain < 0???");
-	return rb_ary_new();
-    }
-    ary = rb_ary_new2(num);
-    for(i = 0; i < num; i++) {
-	x509 = sk_X509_value(chain, i);
-	rb_ary_push(ary, ossl_x509_new(x509));
-    }
-
-    return ary;
+    chain = X509_STORE_CTX_get0_chain(ctx);
+    if (!chain)
+        return Qnil; /* Could be an empty array instead? */
+    return ossl_x509_sk2ary(chain);
 }
 
 /*