solvers: workaround apparently broken code in _frobenius()
solvers: workaround apparently broken code in _frobenius()

I don't think if it's fully correct now (series order n=n+1 seems
to be a heuristics), but the original code is meaningless.  Perhaps,
it was assumed that tseries is an Add instance and author (wrongly) do
an attempt to exstract non-Order polynomial part from the tseries.

skirpichev/omg,diofant/diofant