fix potential crash in unixfs directory

[gammazero]

Fix potential crash in sizeBelowThreshold when modifying children in a directory.

Closes #675

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 60.53%. Comparing base (bcf475c) to head (9256fbe).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ipld/unixfs/io/directory.go	72.22%	4 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #798      +/-   ##
==========================================
+ Coverage   60.49%   60.53%   +0.03%     
==========================================
  Files         245      245              
  Lines       31124    31134      +10     
==========================================
+ Hits        18830    18848      +18     
+ Misses      10615    10610       -5     
+ Partials     1679     1676       -3     

Files with missing lines	Coverage Δ
ipld/unixfs/io/directory.go	68.94% <72.22%> (+0.03%)	⬆️

... and 14 files with indirect coverage changes

[gammazero]

Needs test

[lidel]

Triage notes:

• ok to merge as long there is Kubo PR using this PR and the CI there is green (there are autosharding tests in Kubo iirc that should catch any unexpected regression)