fix panic when cannot download FirmwareIndex

[umbynos]

Closes #127

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 11.04%. Comparing base (c061e88) to head (1a77a29).
⚠️ Report is 116 commits behind head on main.

Files with missing lines	Patch %	Lines
cli/firmware/list.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            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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[per1234]

Fixes #127 for me.
Thanks Umberto!