backtrace_frame should handle assignment to element zero for compatibility

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1799

Reported by Martin Stjernholm mast@roxen.com

In e.g. Roxen there's code that does this:

    foreach (reverse (err[1]), mixed ent) {
      string file, func, descr;
      int line;
      if (arrayp (ent)) {
	if (sizeof (ent) && stringp (ent[0]))
	  if (ent[0][..sizeof (cwd) - 1] == cwd)
	    file = ent[0] = ent[0][sizeof (cwd)..];