Fix native Exception::setPreviousChain() fatal

Fixes a fatal in native Exception, which is missing parentheses on this method call.

hphp/system/php/lang/Exception.php

@@ -84,7 +84,7 @@ class Exception {
     $next = $cur->getPrevious();
     while ($next instanceof Exception) {
       $cur = $next;
-      $next = $cur->getPrevious;
+      $next = $cur->getPrevious();
     }
     $cur->setPrevious($previous);
   }

hphp/test/quick/exception_set_previous_chain.php

@@ -0,0 +1,28 @@
+<?php
+
+$e0 = new Exception('0');
+$e1 = new Exception('1');
+$e2 = new Exception('2');
+$e1->setPrevious($e2);
+$e0->setPrevious($e1);
+
+$eA = new Exception('A');
+$eB = new Exception('B');
+$eC = new Exception('C');
+$eB->setPrevious($eC);
+$eA->setPrevious($eB);
+
+$e0->setPreviousChain($eA);
+
+$proper_order = '012ABC';
+$actual_order = $e0->getMessage();
+
+$cur = $e0;
+while ($cur->getPrevious() !== null) {
+  $cur = $cur->getPrevious();
+  $actual_order .= $cur->getMessage();
+}
+
+$result = $actual_order === $proper_order ? 'pass' : 'fail';
+
+echo "{$result}\n";

hphp/test/quick/exception_set_previous_chain.php.expect

@@ -0,0 +1 @@
+pass