Add a check against negative support of degrees of freedom

Co-authored-by: Ricardo <[email protected]>

pymc3/distributions/multivariate.py

@@ -41,7 +41,7 @@
 
 from pymc3.aesaraf import floatX, intX
 from pymc3.distributions import transforms
-from pymc3.distributions.continuous import ChiSquared, Normal
+from pymc3.distributions.continuous import ChiSquared, Normal, assert_negative_support
 from pymc3.distributions.dist_math import bound, factln, logpow
 from pymc3.distributions.distribution import Continuous, Discrete
 from pymc3.distributions.special import gammaln, multigammaln
@@ -342,6 +342,7 @@ def dist(cls, nu, Sigma=None, mu=None, cov=None, tau=None, chol=None, lower=True
         nu = at.as_tensor_variable(floatX(nu))
         mu = at.as_tensor_variable(floatX(mu))
         cov = quaddist_matrix(cov, chol, tau, lower)
+        assert_negative_support(nu, "nu", "MvStudentT")
         return super().dist([nu, mu, cov], **kwargs)
 
     def logp(value, nu, mu, cov):