Fixed bug in MaxNLocator.bin_boundaries

[bblay]

import numpy.random
import numpy.ma as ma
import matplotlib.pyplot as plt
a = numpy.random.uniform(size=(10,10)) + 280
a2 = ma.MaskedArray(a)
plt.contour(a2)  # Fails

ValueError: zero-size array to minimum.reduce without identity

Fixed a bug where MaxNLocator.bin_boundaries() was changing QuadContourSet.zmin and zmax in place, causing an empty levels list, and therefore the above exception.