Stack overflow when calling constructor with only first type parameter

Had once forgotten to add the second type parameter but I was met with a stack overflow:
```
julia> FixedDecimal{Int8}(5)
ERROR: StackOverflowError:
Stacktrace:
[1] convert(#unused#::Type{FixedDecimal{Int8}}, x::Int64)
@ Base ./number.jl:7
[2] (FixedDecimal{Int8})(x::Int64)
@ FixedPointDecimals ~/.julia/packages/FixedPointDecimals/GFltp/src/FixedPointDecimals.jl:103
--- the last 2 lines are repeated 39990 more times ---
[79983] convert(#unused#::Type{FixedDecimal{Int8}}, x::Int64)
@ Base ./number.jl:7
```