message: Fix NPE
  Summary:
  Bug introduced in {D8054678}. I'd forgotten that DexPosition::file
  could be null.
diff: @@ -937,7 +937,12 @@ std::string show(const DexDebugInstruction* insn) {
  }
  std::ostream& operator<<(std::ostream& o, const DexPosition& pos) {
  - o << *pos.file << ":" << pos.line;
  + if (pos.file == nullptr) {
  + o << "Unknown source";
  + } else {
  + o << *pos.file;
  + }
  + o << ":" << pos.line;
  return o;
  }
  