Fix DynamicLayoutMap index issue

[Kyle-Ye]

A follow up fix after #579

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.38%. Comparing base (6ce7922) to head (a37df6a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nSwiftUICore/Layout/Dynamic/DynamicLayoutMap.swift	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
+ Coverage   27.59%   31.38%   +3.78%     
==========================================
  Files         526      526              
  Lines       30629    30633       +4     
==========================================
+ Hits         8453     9615    +1162     
+ Misses      22176    21018    -1158     

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