SegmentTermEnum.next() doesn't maintain prevBuffer at end

When you're iterating a SegmentTermEnum and you go past the end of the docs, you end up with a state where the nextBuffer = null and the prevBuffer is the penultimate term, not the last term. This patch fixes it. (It's also required for my Prefetching bug LUCENE-506)

Index: java/org/apache/lucene/index/SegmentTermEnum.java
===================================================================
— java/org/apache/lucene/index/SegmentTermEnum.java (revision 382121)
+++ java/org/apache/lucene/index/SegmentTermEnum.java (working copy)
@@ -109,6 +109,7 @@
/** Increments the enumeration to the next element. True if one exists.*/
public final boolean next() throws IOException {
if (position++ >= size - 1)

{ + prevBuffer.set(termBuffer); termBuffer.reset(); return false; }