`Base.show(io::IO, x::Level)` error

Hi, thank you for this wonderful package!

Here's the thing, I can construct a `Level`:
```
y = Unitful.Level{Unitful.Decibel,1}(2);
```

But I can't show it:
```
julia> y
Error showing value of type Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64}:
ERROR: undefined behavior. Please file an issue with the code needed to reproduce.
Stacktrace:
[1] error(s::String)
@ Base ./error.jl:35
[2] isrootpower_dim(y::Unitful.Dimensions{()})
@ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:167
[3] isrootpower(y::Int64)
@ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:166
[4] tolog(L::Type{Unitful.LogInfo{:Decibel, 10, 10}}, S::Int64, x::Float64)
@ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:82
[5] ustrip(x::Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64})
@ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:162
[6] show(io::IOContext{Base.TTY}, x::Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64})
@ Unitful ~/.julia/packages/Unitful/dHMk1/src/logarithm.jl:314
[7] show(io::IOContext{Base.TTY}, ::MIME{Symbol("text/plain")}, x::Level{Unitful.LogInfo{:Decibel, 10, 10}, 1, Int64})
@ Base.Multimedia ./multimedia.jl:47
...
```

A similar issue exists with:
```
x1 = @dB (-2u"V")/1u"V";
```
where I can construct it, but cannot show it.
Here, the issue is probably due to logarithmic scales not allowing negative values. In fact, this works perfectly:
```
julia> x2 = @dB (2u"V")/1u"V"
6.020599913279624 dBV
```
But then, the issue is that that `x1` construction shouldn´t probably be allowed at all.

Start in src/logarithm.jl, especially the show method around line 314 and the logarithmic helpers around lines 82 and 162-167. Reproduce the Level and @dB examples from the issue, then inspect existing tests for logarithmic quantities. Done means the negative-value behavior is deliberate and covered, and displaying valid Level values no longer triggers the reported error.

Written by the indexing model from the issue text.