Free the VarEnv the correct way

detach, rather than destroy.

hphp/runtime/ext/ext_continuation.cpp

@@ -63,7 +63,7 @@ c_Continuation::~c_Continuation() {
   contLocal->m_type = KindOfNull;
 
   if (ar->hasVarEnv()) {
-    VM::VarEnv::destroy(ar->getVarEnv());
+    ar->getVarEnv()->detach(ar);
   } else {
     frame_free_locals_inl(ar, m_vmFunc->numLocals());
   }

hphp/runtime/vm/hhbc.cpp

@@ -696,7 +696,7 @@ std::string instrToString(const Opcode* it, const Unit* u /* = NULL */) {
 #define READOFF() do {                                              \
   Offset _value = *(Offset*)it;                                     \
   out << " " << _value;                                             \
-  if (u != nullptr) {                                                  \
+  if (u != nullptr && _value >= 0) {                                \
     out << " (" << u->offsetOf(iStart + _value) << ")";             \
   }                                                                 \
   it += sizeof(Offset);                                             \