KotlinNullPointerException when calling set of PolygonShape

example:

```
val vecs: Array = Array(8) { Vec2(1F, 1F) }
PolygonShape().set(vecs, vecs.size)
```

issue in org.jbox2d.collision.shapes.PolygonShape:
```
// Perform welding and copy vertices into local buffer.
val ps = if (vecPool != null)
vecPool[Settings.maxPolygonVertices]
else
arrayOfNulls(Settings.maxPolygonVertices)
var tempCount = 0
for (i in 0 until n) {
val v = verts[i]
var unique = true
for (j in 0 until tempCount) {
if (MathUtils.distanceSquared(v, ps[j]!!) < 0.5f * Settings.linearSlop) {
unique = false
break
}
}

if (unique) {
ps[tempCount++]!!.set(v)
}
```

ps will be an array of nulls. Unique is true in the first run, since tempCount is 0

Start in org.jbox2d.collision.shapes.PolygonShape, at the set implementation and the temporary ps buffer shown in the report. Reproduce the supplied Vec2 example and inspect the first unique-vertex path. Done means calling set no longer raises KotlinNullPointerException for that input; check nearby shape behavior for a regression test if the repository provides one.

Written by the indexing model from the issue text.