Fix upcall_vec_append self appending

[msullivan]

Fix bug #498.

[brson]

This doesn't quite get there. If you modify the test case like so:

    let vec[int] a = [0];
    auto i = 20;
    auto expected_len = 1u;
    while (i > 0) {
        log_err vec::len(a);
        assert (vec::len(a) == expected_len);
        a += a;
        i -= 1;
        expected_len *= 2u;
    }

Then it's evident that the vector never grows. I suspect it's because src->fill == 0. I'd also like to see an identical test case for strings as well since off-by-1 errors are easy to miss here.

[brson]

That is, if strings actually use this same code path then a string test would also be good. Not sure if that's the case or not.

[msullivan]

The bug is that the if (src == dst) conditional assumes that the "regrow vector" path was taken in vec_grow. I'll push a fix in a minute.

Yeah, strings use the same code path.

[msullivan]

OK, I think we should be good now.

[brson]

Awesome, thanks!