Fix bug in previous PR for comparisons that underflow (large negatives)

[NHDaly]

In these cases, we were previously returning the wrong result. 😓

julia> @test typemin(Int8) < FD{Int8, 2}(-1) < typemax(Int8)
Test Failed at REPL[27]:1
  Expression: typemin(Int8) < FD{Int8, 2}(-1) < typemax(Int8)
   Evaluated: -128 < FixedDecimal{Int8,2}(-1.00) < 127

julia> @test typemin(FD{Int8, 1}) < typemin(FD{Int8,2})
Test Failed at REPL[34]:1
  Expression: typemin(FD{Int8, 1}) < typemin(FD{Int8, 2})
   Evaluated: FixedDecimal{Int8,1}(-12.8) < FixedDecimal{Int8,2}(-1.28)

Now those tests pass.

[mcmcgrath13]

thank you!!