BUG: incorrect limits in discrete ppf, fixes ticket:1802, see also ticket:1803
BUG: remove two sources of endless loop (self.a==self.b, and initial a,b are not integer valued)
TST: add test for .a == .b case
REF: add check that updating doesn't stop to prevent endless loops
MAINT: remove unused variables from stats.distributions.
Merge branch 'discrete-ppf-1802' into master.

Reviewed as https://github.com/scipy/scipy/pull/394