Issue #2172 - add Zone null checks.

js/src/gc/Heap.h

@@ -1280,8 +1280,10 @@ JS::Zone*
 TenuredCell::zone() const
 {
     JS::Zone* zone = arena()->zone;
-    MOZ_ASSERT(CurrentThreadCanAccessZone(zone));
-    return zone;
+    if (CurrentThreadCanAccessZone(zone))
+        return zone;
+
+    return nullptr;
 }
 
 JS::Zone*

js/src/gc/Marking.cpp

@@ -788,7 +788,8 @@ ShouldMark<JSObject*>(GCMarker* gcmarker, JSObject* obj)
     // Don't mark things outside a zone if we are in a per-zone GC. It is
     // faster to check our own arena, which we can do since we know that
     // the object is tenured.
-    return obj->asTenured().zone()->shouldMarkInZone();
+    Zone* zone = obj->asTenured().zone();
+    return (zone && zone->shouldMarkInZone());
 }
 
 template <typename T>

js/src/vm/Runtime.cpp

@@ -877,6 +877,9 @@ js::CurrentThreadCanAccessRuntime(const JSRuntime* rt)
 bool
 js::CurrentThreadCanAccessZone(Zone* zone)
 {
+    if (!zone)
+        return false;
+
     if (CurrentThreadCanAccessRuntime(zone->runtime_))
         return true;