Suppress invalid argument warnings in inverse Mollweide projection

The Mollweide inverse projection generates Numpy invalid value warnings for pixels outside of the projection. For example, the following code:

    from matplotlib import pyplot as plt
    plt.subplot(111, projection='mollweide')
    plt.savefig('test.png')

produces this warning:

    .../matplotlib/projections/geo.py:489:
    RuntimeWarning: invalid value encountered in arcsin
      theta = np.arcsin(y / np.sqrt(2))

This patch silences the warning by wrapping the inverse transformation in a `np.errstate` context.