Fix a crash if an exception is thrown in a constructor's surprise check

The unwinder assumed that if the actrec's constructor flag
was set, then there must be a $this. But the $this is cleared during
the return sequence.

hphp/runtime/vm/bytecode.cpp

@@ -982,8 +982,7 @@ UnwindStatus Stack::unwindFrag(ActRec* fp, int offset,
   // count starts over for the caller frame.
   fault.m_handledCount = 0;
 
-  if (fp->isFromFPushCtor()) {
-    assert(fp->hasThis());
+  if (fp->isFromFPushCtor() && fp->hasThis()) {
     fp->getThis()->setNoDestruct();
   }
 

hphp/test/quick/constructor_throw.php

@@ -0,0 +1,21 @@
+<?php
+
+class X {
+  function __construct() {
+  }
+}
+
+function handler($kind, $name) {
+  if ($kind == 'exit' && $name == 'X::__construct') throw new Exception;
+}
+
+function test() {
+  fb_setprofile('handler');
+  try {
+    new X;
+  } catch (Exception $e) {
+    echo "ok\n";
+  }
+}
+
+test();

hphp/test/quick/constructor_throw.php.expect

@@ -0,0 +1 @@
+ok