Be more robust against buggy plugins missing their descriptors.
Also fixing upstream, but best to be defensive here too.
… WARNING	c.c.j.support.SupportPlugin#writeBundle: Could not attach 'nodes.md' to support bundle
java.lang.AssertionError: class hudson.plugins.swarm.SwarmSlave$1 is missing its descriptor
	at jenkins.model.Jenkins.getDescriptorOrDie(Jenkins.java:1182)
	at hudson.model.AbstractDescribableImpl.getDescriptor(AbstractDescribableImpl.java:35)
	at com.cloudbees.jenkins.support.impl.AboutJenkins$2.printTo(AboutJenkins.java:281)
	at com.cloudbees.jenkins.support.api.PrintedContent.writeTo(PrintedContent.java:47)
	at com.cloudbees.jenkins.support.SupportPlugin.writeBundle(SupportPlugin.java:335)
	at com.cloudbees.jenkins.support.SupportPlugin.writeBundle(SupportPlugin.java:253)
	at com.cloudbees.jenkins.support.SupportPlugin$PeriodicWorkImpl$1.run(SupportPlugin.java:750)
	at java.lang.Thread.run(Thread.java:701)