More robust semaphores

Fixes #181

Author: codefrau

vm.primitives.js

@@ -1914,11 +1914,11 @@ Object.subclass('Squeak.Primitives',
         this.semaphoresToSignal.push(semaIndex);
     },
     signalExternalSemaphores: function() {
-        var semaphores = this.vm.specialObjects[Squeak.splOb_ExternalObjectsArray].pointers,
+        var semaphores = this.vm.specialObjects[Squeak.splOb_ExternalObjectsArray].pointers || [],
             semaClass = this.vm.specialObjects[Squeak.splOb_ClassSemaphore];
         while (this.semaphoresToSignal.length) {
             var semaIndex = this.semaphoresToSignal.shift(),
-                sema = semaphores[semaIndex - 1];
+                sema = semaphores[semaIndex - 1] || this.vm.nilObj;
             if (sema.sqClass == semaClass)
                 this.synchronousSignal(sema);
         }