Fix crash with only one new match

[zenangst]

Fix crash when trying to diff two collection where the new one only contains one item.
When this happens the algorithm went out of bounds as it moves the offset around.

[codecov-io]

Codecov Report

Merging #12 into master will decrease coverage by 0.45%.
The diff coverage is 52.38%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   32.52%   32.07%   -0.46%     
==========================================
  Files          13       13              
  Lines         578      583       +5     
==========================================
- Hits          188      187       -1     
- Misses        390      396       +6

Impacted Files	Coverage Δ
Source/Shared/Algorithm.swift	90.98% <52.38%> (-4.75%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 775fbf5...c286d97. Read the comment docs.