server: fix crash with invalid SETUP request (bluenviron/mediamtx#4025)

[aler9]

Fixes bluenviron/mediamtx#4025

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.40%. Comparing base (d7efbd0) to head (bd3b3c3).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/liberrors/server.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #652      +/-   ##
==========================================
+ Coverage   82.31%   82.40%   +0.09%     
==========================================
  Files         116      116              
  Lines       11784    11796      +12     
==========================================
+ Hits         9700     9721      +21     
+ Misses       1696     1689       -7     
+ Partials      388      386       -2     

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