[fix] Tree::_grow_arena() should start from size and not position

[biojppm]

fixes #288

[codecov]

Codecov Report

Merging #290 (ec28a2b) into master (d44a9b9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #290   +/-   ##
=======================================
  Coverage   97.11%   97.11%           
=======================================
  Files          77       77           
  Lines       17124    17147   +23     
=======================================
+ Hits        16630    16653   +23     
  Misses        494      494           

Impacted Files	Coverage Δ
src/c4/yml/tree.cpp	94.72% <100.00%> (ø)
src/c4/yml/tree.hpp	98.64% <100.00%> (-0.01%)	⬇️
test/test_case.cpp	87.18% <100.00%> (+0.13%)	⬆️
test/test_tree.cpp	99.89% <100.00%> (+<0.01%)	⬆️

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