fix panic when cannot download FirmwareIndex

[umbynos]

Closes #127

[codecov-commenter]

Codecov Report

Merging #128 (1a77a29) into main (c061e88) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   11.04%   11.04%   -0.01%     
==========================================
  Files          21       21              
  Lines        1810     1811       +1     
==========================================
  Hits          200      200              
- Misses       1551     1552       +1     
  Partials       59       59              

Flag	Coverage Δ
unit	11.04% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
cli/firmware/list.go	0.00% <0.00%> (ø)

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[per1234]

Fixes #127 for me.
Thanks Umberto!