Don't hang during hash calibration for small minimums

Before this fix the installer would get stuck or run very long
inside the hash calibration loop for:

  install.py ... --minimum-password-hash-time 0.01
  install.py ... --minimum-password-hash-time 0.00001
  install.py ... --minimum-password-hash-time 0

Introduced by 2cba89d ("Fix float division by zero")