GF_Bloc_ImFreq::density() returns NaN for bosonic GF's

[krivenko]

GF_Bloc_ImFreq::density() constantly returns 'not a number' values for a bosonic GF.
There is a line in the code (triqs/gf_local/GF_Bloc_ImFreq.cpp, line 63):

dens_part(n1,n2)+= data(n1,n2,om)  -  (a1/(om1 - b1) + a2 / (om1-b2) + a3/(om1-b3));

which does a Matsubara summation of the GF's data and explicitly subtracts a tail contribution. But term (a1/(om1 - b1) (with b1=0) is affected by a zero-frequency problem.