PipelineMain: only close initialised compilers
PipelineMain: only close initialised compilers

The stack

    java.nio.file.NoSuchFileException: /tmp/scala.picklecache2320636410138511747/tmp/pipelineBase4140053099421013810/projects/b5/p1/target.jar
    	at sun.nio.fs.UnixException.translateToIOException(UnixException.java:86)
    	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
    	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
    	at sun.nio.fs.UnixPath.toRealPath(UnixPath.java:837)
    	at scala.tools.nsc.PipelineMainClass.$anonfun$newCompiler$1(PipelineMain.scala:605)
    	at scala.collection.immutable.List.map(List.scala:291)
    	at scala.tools.nsc.PipelineMainClass.newCompiler(PipelineMain.scala:602)
    	at scala.tools.nsc.PipelineMainClass$Task.liftedTree1$1(PipelineMain.scala:417)
    	at scala.tools.nsc.PipelineMainClass$Task.compiler$lzycompute(PipelineMain.scala:416)
    	at scala.tools.nsc.PipelineMainClass$Task.compiler(PipelineMain.scala:416)
    	at scala.tools.nsc.PipelineMainClass.$anonfun$process$58(PipelineMain.scala:246)
    	at scala.tools.nsc.PipelineMainClass.$anonfun$process$58$adapted(PipelineMain.scala:246)
    	at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
    	at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
    	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
    	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
    	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

in #8914 (https://travis-ci.org/github/scala/scala/builds/678537798)
indicates we're still suffering from race conditions in PipelineMain.

But from the line numbers (PipelineMain.scala:246) it seems to be
triggered while initialising the compiler from a `p.compiler.close()`
call, ironically.  So let's avoid that first and see if it happens
again.

scala/scala,lrytz/scala,lrytz/scala,scala/scala,scala/scala,scala/scala,lrytz/scala,lrytz/scala,lrytz/scala,scala/scala,scala/scala,lrytz/scala