Fix crash in Folder, GNU and Crates backends

[Zlopez]

Signed-off-by: Michal Konečný [email protected]

[codecov-io]

Codecov Report

Merging #837 into master will decrease coverage by 0.02%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
- Coverage   97.36%   97.34%   -0.03%     
==========================================
  Files          61       61              
  Lines        3650     3654       +4     
  Branches      492      494       +2     
==========================================
+ Hits         3554     3557       +3     
  Misses         58       58              
- Partials       38       39       +1

Impacted Files	Coverage Δ
anitya/lib/backends/crates.py	100% <100%> (ø)	⬆️
anitya/lib/backends/folder.py	96.96% <100%> (ø)	⬆️
anitya/lib/backends/gnu.py	96.77% <75%> (-3.23%)	⬇️

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