Fix memory leak in ectest

Fixes #8462

(cherry picked from commit 81d61a62faa809e6c51f5fc2b86fb0d31146fd5e)
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8466)

test/ectest.c

@@ -1466,6 +1466,7 @@ static int underflow_test(void)
     BN_CTX_end(ctx);
     EC_POINT_free(P);
     EC_POINT_free(Q);
+    EC_POINT_free(R);
     EC_GROUP_free(grp);
     BN_CTX_free(ctx);