Fix potential memory leak in hs_helper_build_intro_point().

This patch fixes a potential memory leak in
hs_helper_build_intro_point() where a `goto done` is called before the
`intro_point` variable have been assigned to the value of the `ip`
variable.

See: Coverity CID 1437460
See: Coverity CID 1437456

src/test/hs_test_helpers.c

@@ -85,6 +85,9 @@ hs_helper_build_intro_point(const ed25519_keypair_t *signing_kp, time_t now,
 
   intro_point = ip;
  done:
+  if (intro_point == NULL)
+    tor_free(ip);
+
   return intro_point;
 }