fix concurrent map iteration and write

[FZambia]

fatal error: concurrent map iteration and map write

goroutine 96 [running]:
internal/runtime/maps.fatal({0x102060d5f?, 0x14000100598?})
	/opt/homebrew/Cellar/go/1.24.2/libexec/src/runtime/panic.go:1058 +0x20
internal/runtime/maps.(*Iter).Next(0x14000a05ed0?)
	/opt/homebrew/Cellar/go/1.24.2/libexec/src/internal/runtime/maps/table.go:683 +0x94
github.com/centrifugal/centrifuge.(*RedisBroker).runShard.func2()
	/Users/alexander.emelin/centrifugal/centrifuge/broker_redis.go:376 +0xc0

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.56%. Comparing base (d423464) to head (1b2263c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
broker_redis.go	96.87%	1 Missing ⚠️
presence_redis.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
+ Coverage   83.39%   83.56%   +0.16%     
==========================================
  Files          39       39              
  Lines        8978     8997      +19     
==========================================
+ Hits         7487     7518      +31     
+ Misses       1126     1117       -9     
+ Partials      365      362       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.