Fix newly found linter issues

[S7evinK]

Fixes the issues found in https://github.com/matrix-org/dendrite/actions/runs/5155539352/jobs/9285342056#step:5:22.
Only naked returns in longer functions.

[codecov]

Codecov Report

Patch coverage: 20.00% and no project coverage change.

Comparison is base (ea6b368) 66.18% compared to head (f6496f2) 66.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3099   +/-   ##
=======================================
  Coverage   66.18%   66.19%           
=======================================
  Files         500      500           
  Lines       53915    53916    +1     
=======================================
+ Hits        35686    35691    +5     
+ Misses      14633    14631    -2     
+ Partials     3596     3594    -2     

Flag	Coverage Δ
unittests	50.02% <20.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
roomserver/internal/perform/perform_peek.go	44.95% <0.00%> (ø)
syncapi/streams/stream_pdu.go	82.13% <0.00%> (ø)
syncapi/routing/messages.go	66.84% <25.00%> (+0.08%)	⬆️
userapi/storage/shared/storage.go	77.34% <50.00%> (ø)

... and 7 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.