Don't crash reversing empty Vectors

In a new, empty vector start and end are null.

hphp/runtime/ext/ext_collections.cpp

@@ -405,6 +405,7 @@ void c_Vector::t_sort(CVarRef col /* = null */) {
 }
 
 void c_Vector::t_reverse() {
+  if (m_size <= 1) return;
   TypedValue* start = m_data;
   TypedValue* end = m_data + m_size - 1;
   for (; start < end; ++start, --end) {

hphp/test/slow/collection_classes/empty_vector.php

@@ -0,0 +1,9 @@
+<?php
+
+function test() {
+  $v = new Vector;
+  $v->reverse();
+  var_dump($v);
+}
+
+test();

hphp/test/slow/collection_classes/empty_vector.php.expect

@@ -0,0 +1,2 @@
+object(Vector)#1 (0) {
+}