truncnorm does not work if b < a in scipy.stats

[chrisb83]

truncnorm does not check that a < b. If b > a, one can produce rvs but the pdf and cdf do not work.
Should be easy to fix using _argcheck.

Reproducing code example:

from scipy import stats
import numpy as np

x = np.arange(1, 2, 0.1)
stats.truncnorm(2, 1).pdf(x) # all 0
stats.truncnorm(2, 1).cdf(1) # 1.0
stats.truncnorm(2, 1).cdf(0.999) # 0.0
stats.truncnorm(2, 1).rvs(size=5, random_state=123) 
# array([1.1880652 , 1.54116815, 1.61108676, 1.29490424, 1.17234318])

This does not make sense.

Scipy/Numpy/Python version information:

1.0.0 1.14.1 sys.version_info(major=3, minor=5, micro=4, releaselevel='final', serial=0)

[akahard2dj]

@chrisb83
I fixed this defect by adding condition in _argcheck.
#9174