Best-effort: fix compiler crash when using betasty with missing java classfiles

[jchyb]

Fixes the

exception occurred while extractSemanticDBExtractSemanticInfo /Users/tgodzik/Documents/dotty/compiler/test/dotty/tools/dotc/classpath/JrtClassPathTest.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.


     while compiling: /Users/tgodzik/Documents/dotty/compiler/test/dotty/tools/dotc/classpath/JrtClassPathTest.scala
        during phase: parser
                mode: Mode(ImplicitsEnabled,ReadPositions,SafeNulls)
     library version: version 2.13.16
    compiler version: version 3.6.4-RC1
            settings: -Wconf List(cat=deprecation&origin=scala\.collection\.mutable\.AnyRefMap.*:s) -Wsafe-init true -Xsemanticdb true -Ybest-effort true -Yexplicit-nulls true -Ywith-best-effort-tasty true -classpath /Users/tgodzik/Documents/dotty/compiler/

crash when best effort is used in the compiler on erroring code (where we lose access to java classifies)

[tgodzik]

LGTM!