Crash when creating a gather label that is also a called function (and vice-versa)

Ran into this very small issue that had me confused for a while, so I thought I'd bring it up.

```
=== function gather_test(x) ===
~ return 1

=== knot_test ===
= stitch_test
- (gather_test)
{gather_test("hey")}
->END
```

This crashes the compiler and requires you to fix the problem and then restart/reload Inky to get back to normal. 😣

If you create the gather label BEFORE the function is called in the ink file, you get a nice error saying the gather name has already been used. But if you create the gather after a function has been called or call the function after the gather has been created, you get a crash.

Apologies if this is too much of an edge case but it was frustrating to debug.