message: elle.Backtrace: avoid log of 0
diff: @@ -238,11 +238,11 @@ namespace elle
  std::ostream&
  operator<< (std::ostream& out, const Backtrace& bt)
  {
  + auto const& fs = bt.frames();
  unsigned i = 0;
  - // Visual expects a float ... don't ask.
  - size_t const width = std::log10(float(bt.frames().size())) + 1;
  + size_t const width = (fs.empty() ? 0 : std::log10(fs.size())) + 1;
  auto const fmt = "#%-" + std::to_string(width) + "d %s\n";
  - for (const auto& f: bt.frames())
  + for (const auto& f: fs)
  fprintf(out, fmt, i++, f);
  return out;
  }
  