fix(mock): prevent NPE when module definition outside of it.

Author: mhevery

Diff for: src/ngMock/angular-mocks.js

@@ -1782,7 +1782,7 @@ window.jstestdriver && (function(window) {
         try {
           injector.invoke(blockFns[i] || angular.noop, this);
         } catch (e) {
-          if(e.stack) e.stack +=  '\n' + errorForStack.stack;
+          if(e.stack && errorForStack) e.stack +=  '\n' + errorForStack.stack;
           throw e;
         } finally {
           errorForStack = null;