Fix NPE regression in Vector.prependedAll

Also make a corresponding fix to appenededAll. This isn't exercised
by the test case though.

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

@@ -193,13 +193,15 @@ sealed abstract class Vector[+A] private[immutable] (private[immutable] final va
   override def prepended[B >: A](elem: B): Vector[B] = super.prepended(elem)
   override def prependedAll[B >: A](prefix: collection.IterableOnce[B]): Vector[B] = {
     val k = prefix.knownSize
-    if(k == 0) this
+    if (k == 0) this
+    else if (k < 0) super.prependedAll(prefix)
     else prependedAll0(prefix, k)
   }
 
   override final def appendedAll[B >: A](suffix: collection.IterableOnce[B]): Vector[B] = {
     val k = suffix.knownSize
-    if(k == 0) this
+    if (k == 0) this
+    else if (k < 0) super.appendedAll(suffix)
     else appendedAll0(suffix, k)
   }
 

test/files/run/t12564.scala

@@ -0,0 +1,14 @@
+object Test extends App {
+
+  val vector1 = Vector.fill(1000)(0)
+
+  // Some prepending, manifesting the bug seems to require 2 separate prepends
+  val vector2 = List.fill(25)(1) ++: 2 +: vector1
+
+  // Now append for fun and profit
+  val vector3 = vector2 ++ List.fill(40)(3)
+
+  // Any iteration will do although might hit NullPointerException in different place
+  val vector4 = vector3.collect { case n => n }
+  assert(vector3 == vector4)  
+}