Fix user defined sorting

copy paste error crept in a few weeks ago, breaking the
case of a user-defined function that returns boolean.

hphp/runtime/base/array/sort_helpers.h

@@ -273,7 +273,7 @@ struct ElmUCompare {
       Variant ret2;
       tvDup(acc.getValue(right).asTypedValue(), args+0);
       tvDup(acc.getValue(left).asTypedValue(), args+1);
-      g_vmContext->invokeFuncFew(ret.asTypedValue(), *ctx,
+      g_vmContext->invokeFuncFew(ret2.asTypedValue(), *ctx,
                                  2, args);
       if (ret2.isBoolean()) {
         return ret2.toBoolean();

hphp/test/slow/array_functions/usort.php

@@ -0,0 +1,10 @@
+<?php
+
+function less($a, $b) { return $a < $b; }
+
+function main($a) {
+  usort($a, 'less');
+  var_dump($a);
+}
+
+main(array(1,2));

hphp/test/slow/array_functions/usort.php.expect

@@ -0,0 +1,6 @@
+array(2) {
+  [0]=>
+  int(2)
+  [1]=>
+  int(1)
+}