boids: population needs to be an integer for slicing.

extra/boids/boids.factor

@@ -96,7 +96,7 @@ M: range-observer model-changed
 
 :: set-population ( n boids-gadget -- )
     boids-gadget [
-        dup length n - dup 0 >
+        dup length n - >integer dup 0 >
         [ head* ]
         [ neg random-boids append ] if
     ] change-boids drop ;