Pad short parameter names before using File.createTempFile

[jglick]

Otherwise can get an error:

Also:   hudson.remoting.Channel$CallSiteStackTrace: Remote call to remote
		at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1800)
		at hudson.remoting.UserRequest$ExceptionResponse.retrieve(UserRequest.java:357)
		at hudson.remoting.Channel.call(Channel.java:1001)
		at hudson.FilePath.act(FilePath.java:1070)
		at hudson.FilePath.act(FilePath.java:1059)
		at hudson.FilePath.createTempFile(FilePath.java:1419)
		at io.jenkins.plugins.file_parameters.AbstractFileParameterValue.createTempFile(AbstractFileParameterValue.java:75)
		at io.jenkins.plugins.file_parameters.FileParameterWrapper.setUp(FileParameterWrapper.java:89)
		at org.jenkinsci.plugins.workflow.steps.CoreWrapperStep$Execution2.doStart(CoreWrapperStep.java:116)
		at org.jenkinsci.plugins.workflow.steps.GeneralNonBlockingStepExecution.lambda$run$0(GeneralNonBlockingStepExecution.java:77)
		at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
java.lang.IllegalArgumentException: Prefix string too short
	at java.io.File.createTempFile(File.java:2003)
	at hudson.FilePath$CreateTempFile.invoke(FilePath.java:1434)
	at hudson.FilePath$CreateTempFile.invoke(FilePath.java:1424)
	at hudson.FilePath$FileCallableWrapper.call(FilePath.java:3122)
	at hudson.remoting.UserRequest.perform(UserRequest.java:211)
	at hudson.remoting.UserRequest.perform(UserRequest.java:54)
	at hudson.remoting.Request$2.run(Request.java:375)
	at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:73)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)