We still hardcode paths, but this is better.

- Initialize lengthOfInputFile and bufferSize to 0 in case input isn't
  found.

- Pull call to fopen out of ifdefs, so a future patch can just rip out
  the hardcoded path parts.

- If input _is_ found, then don't leak the file descriptor when the
  FileReaderThread gets deleted.

- If input isn't found, emit a sensible error message and bail in the
  constructor when the file isn't found. Have subsequent calls that
  try to use the FileReaderThread fail instead of crash.

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>