Fix bug in fusion tree iterator for multiple fusion case

[Jutho]

Fixes #192 .

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.01%. Comparing base (00eb107) to head (598d85c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/fusiontrees/iterator.jl	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
- Coverage   82.04%   82.01%   -0.04%     
==========================================
  Files          42       42              
  Lines        5368     5370       +2     
==========================================
  Hits         4404     4404              
- Misses        964      966       +2     

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