panic: using both animate-interval and force-appendix

[metafeather]

Related to: #1206

The fix does not appear to be in a subsequest release as I ran into this with tooltip too

t.d2

a: {
  tooltip: some text
}

d2 --version
> 0.7.1

d2 --animate-interval=120 --force-appendix t.d2
> panic: runtime error: slice bounds out of range [:-1]