[MRG] Fix panic bug in sourmash sketch dna with bad input and --check-sequence

[mr-eyes]

A PR that fixes #1700

[mr-eyes]

@ctb I think this resolves the issue and is ready for review.

[codecov]

Codecov Report

Merging #1702 (46c4f63) into latest (222f54b) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           latest    #1702      +/-   ##
==========================================
+ Coverage   82.61%   82.63%   +0.01%     
==========================================
  Files         113      113              
  Lines       11996    11994       -2     
  Branches     1513     1513              
==========================================
  Hits         9911     9911              
+ Misses       1830     1828       -2     
  Partials      255      255              

Flag	Coverage Δ
python	90.10% <ø> (ø)
rust	66.49% <0.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/signature.rs	61.27% <0.00%> (+0.32%)	⬆️

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[mr-eyes]

re-Renamed to WIP due to a failed rust check.

[ctb]

thank you!