fix nil url deref when handling invalid requests

[database64128]

This fixes the following panic when handling certain invalid requests:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x50 pc=0x5601e7562614]
goroutine 206 [running]:
github.com/bluenviron/gortsplib/v4/pkg/url.(*URL).RTSPPathAndQuery(...)
        github.com/bluenviron/gortsplib/v4@v4.3.0/pkg/url/url.go:81
github.com/bluenviron/gortsplib/v4.(*ServerConn).handleRequestInner(0xc0001a8200, 0xc00064c7c0)
        github.com/bluenviron/gortsplib/v4@v4.3.0/server_conn.go:221 +0x174
github.com/bluenviron/gortsplib/v4.(*ServerConn).handleRequestOuter(0xc0001a8200, 0xc00064c7c0)
        github.com/bluenviron/gortsplib/v4@v4.3.0/server_conn.go:382 +0x67
github.com/bluenviron/gortsplib/v4.(*ServerConn).runInner(0xc0001a8200)
        github.com/bluenviron/gortsplib/v4@v4.3.0/server_conn.go:191 +0x172
github.com/bluenviron/gortsplib/v4.(*ServerConn).run(0xc0001a8200)
        github.com/bluenviron/gortsplib/v4@v4.3.0/server_conn.go:165 +0x348
created by github.com/bluenviron/gortsplib/v4.newServerConn in goroutine 7
        github.com/bluenviron/gortsplib/v4@v4.3.0/server_conn.go:109 +0x3d6

[codecov]

Codecov Report

Merging #458 (e189695) into main (0933bf9) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head e189695 differs from pull request most recent head 42797c7. Consider uploading reports for the commit 42797c7 to get more accurate results

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   84.55%   84.52%   -0.03%     
==========================================
  Files         106      106              
  Lines       10921    10926       +5     
==========================================
+ Hits         9234     9235       +1     
- Misses       1291     1295       +4     
  Partials      396      396              

Files	Coverage Δ
server_conn.go	89.58% <0.00%> (-1.36%)	⬇️

... and 3 files with indirect coverage changes

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[aler9]

Hello, thanks for reporting the issue, i moved the check a little above in order to cover all cases, and i added tests.