Avoid bounds violation with 0-degree Cheb. polynomial.

[isutoris]

Fixes bounds violation when evaluating 0-th degree Chebyshev polynomial by treating the case separately. Should fix #47.

[sglyon]

Thanks for submitting this @isutoris !

Would you be willing to add a tests case that covers your usage (minus the ForwardDiff part)?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9fd30c6). Click here to learn what that means.
The diff coverage is 33.33%.

@@            Coverage Diff            @@
##             master      #48   +/-   ##
=========================================
  Coverage          ?   91.01%           
=========================================
  Files             ?       12           
  Lines             ?     1346           
  Branches          ?        0           
=========================================
  Hits              ?     1225           
  Misses            ?      121           
  Partials          ?        0

Impacted Files	Coverage Δ
src/smol_util.jl	97.69% <33.33%> (ø)
src/cheb.jl	82.07% <33.33%> (ø)

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[isutoris]

Thanks for quick feedback. Yes, adding test seems like a good idea, I will try (although it may take me couple days to get to it).