Fix mistake while switching to new PI constants

From previous pullrequest (https://github.com/assimp/assimp/pull/859),
commit 7c0e40a3f4 introduced the error.

Thanks @mosra for catching it!

Signed-off-by: Squareys <Squareys@googlemail.com>

contrib/poly2tri/poly2tri/sweep/sweep.cc

@@ -231,7 +231,7 @@ void Sweep::FillAdvancingFront(SweepContext& tcx, Node& n)
 
   while (node->next) {
     double angle = HoleAngle(*node);
-    if (angle > PI || angle < -PI_2) break;
+    if (angle > PI_2 || angle < -PI_2) break;
     Fill(tcx, *node);
     node = node->next;
   }