Fix compiler crash when file path is invalid

[noti0na1]

Compiler will crash when file path is invalid and throw NullPointerException, which doesn't contain any useful information.

Example:

sbt:dotty> dotc Xprint:all

Before fix:

java.lang.NullPointerException while compiling Xprint:all
Exception in thread "main" java.lang.NullPointerException
        at dotty.tools.dotc.printing.Formatting$StringFormatter.showArg(Formatting.scala:39)
        at dotty.tools.dotc.printing.Formatting$StringFormatter.treatArg(Formatting.scala:49)
        at dotty.tools.dotc.printing.Formatting$StringFormatter.$anonfun$2(Formatting.scala:62)
        at scala.collection.LazyZip2$$anon$1$$anon$2.next(LazyZipOps.scala:42)
        at scala.collection.mutable.Growable.addAll(Growable.scala:62)
        at scala.collection.mutable.Growable.addAll$(Growable.scala:59)
        at scala.collection.mutable.ArrayBuilder.addAll(ArrayBuilder.scala:69)
        at scala.collection.IterableOnceOps.toArray(IterableOnce.scala:1289)
        at scala.collection.IterableOnceOps.toArray$(IterableOnce.scala:1283)
        at scala.collection.AbstractIterable.toArray(Iterable.scala:920)
        at scala.Array$.from(Array.scala:59)
        at scala.collection.immutable.ArraySeq$.from(ArraySeq.scala:180)
        at scala.collection.immutable.ArraySeq$.from(ArraySeq.scala:171)
        at scala.collection.ClassTagIterableFactory$AnyIterableDelegate.from(Factory.scala:656)
        at scala.collection.BuildFromLowPriority2$$anon$11.fromSpecific(BuildFrom.scala:112)
        at scala.collection.BuildFromLowPriority2$$anon$11.fromSpecific(BuildFrom.scala:109)
        at scala.collection.LazyZip2.map(LazyZipOps.scala:37)
        at dotty.tools.dotc.printing.Formatting$StringFormatter.assemble(Formatting.scala:62)
        at dotty.tools.dotc.core.Decorators$StringInterpolators$.i$extension(Decorators.scala:215)
        at dotty.tools.dotc.Run.compile$$anonfun$1(Run.scala:114)
        at dotty.tools.dotc.reporting.NoExplanation.msg(Message.scala:122)
        at dotty.tools.dotc.reporting.Message.message(Message.scala:80)
        at dotty.tools.dotc.reporting.Message.isNonSensical(Message.scala:92)
        at dotty.tools.dotc.reporting.HideNonSensicalMessages.isHidden(HideNonSensicalMessages.scala:16)
        at dotty.tools.dotc.reporting.AbstractReporter.isHidden(AbstractReporter.scala:8)
        at dotty.tools.dotc.reporting.Reporter.report(Reporter.scala:265)
        at dotty.tools.dotc.reporting.Reporting.echo(Reporter.scala:78)
        at dotty.tools.dotc.core.Contexts$Context.echo(Contexts.scala:78)
        at dotty.tools.dotc.Run.compile(Run.scala:114)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:38)
        at dotty.tools.dotc.Driver.process(Driver.scala:194)
        at dotty.tools.dotc.Driver.process(Driver.scala:163)
        at dotty.tools.dotc.Driver.process(Driver.scala:175)
        at dotty.tools.dotc.Driver.main(Driver.scala:202)
        at dotty.tools.dotc.Main.main(Main.scala)

After fix:

invalid file path: Illegal char <:> at index 6: Xprint:all
1 error found