Fix condition where subtraction can overflow

Author: travisbrown

library/src/scala/collection/immutable/Vector.scala

@@ -286,7 +286,7 @@ private sealed abstract class VectorImpl[+A](_prefix1: Arr1) extends Vector[A](_
     val hi = mmin(until, length)
     val newlen = hi - lo
     if(newlen == length) this
-    else if(newlen <= 0) Vector0
+    else if(hi <= lo) Vector0
     else slice0(lo, hi)
   }
 }