Fix NPE with Jenkins 1.535

After upgrading to Jenkins 1.535, I got the following NPE and fixed it with the suggested patch:

```
java.lang.NullPointerException
	at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:242)
	at hudson.plugins.emailext.ExtendedEmailPublisher.prebuild(ExtendedEmailPublisher.java:222)
	at hudson.model.AbstractBuild$AbstractBuildExecution.preBuild(AbstractBuild.java:815)
	at hudson.model.AbstractBuild$AbstractBuildExecution.preBuild(AbstractBuild.java:810)
	at hudson.model.Build$BuildExecution.doRun(Build.java:142)
	at hudson.plugins.project_inheritance.projects.InheritanceBuild$InheritanceBuildExecution.doRun(InheritanceBuild.java:111)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:562)
	at hudson.model.Run.execute(Run.java:1665)
	at hudson.plugins.project_inheritance.projects.InheritanceBuild.run(InheritanceBuild.java:96)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:246)
```