BUG: signal.filtfilt: make a and b coefficients atleast_1d array

The following crashed before this fix::

    import numpy as np
    from scipy.signal import filtfilt
    x = np.random.randn(100)
    y = filtfilt(np.ones(5)/5, 1, x)
Merge branch 'pr/3701' into master.

Fixes bug in signal.filtfilt.

Reviewed at https://github.com/scipy/scipy/pull/3701