Avoid stackoverflow in defaultgetindex

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 :tada:

Comparison is base (6f0cf81) 70.54% compared to head (aa5e90e) 70.55%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #489 +/- ## ========================================== + Coverage 70.54% 70.55% +0.01% ========================================== Files 81 81 Lines 8477 8485 +8 ========================================== + Hits 5980 5987 +7 - Misses 2497 2498 +1 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/489?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Operators/Operator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/489?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9PcGVyYXRvci5qbA==) | `72.75% <100.00%> (+0.48%)` | :arrow_up: | | [src/Operators/functionals/Evaluation.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/489?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9mdW5jdGlvbmFscy9FdmFsdWF0aW9uLmps) | `87.09% <100.00%> (-0.95%)` | :arrow_down: |

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