fix crash that can happen during publisher override

go.mod

@@ -5,7 +5,7 @@ go 1.15
 require (
 	github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751 // indirect
 	github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d // indirect
-	github.com/aler9/gortsplib v0.0.0-20210516134729-c6ff1e0d0243
+	github.com/aler9/gortsplib v0.0.0-20210516193013-ea6138c28286
 	github.com/asticode/go-astits v0.0.0-00010101000000-000000000000
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/fsnotify/fsnotify v1.4.9

go.sum

@@ -4,8 +4,8 @@ github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d h1:UQZhZ2O0vMHr2c
 github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d/go.mod h1:rBZYJk541a8SKzHPHnH3zbiI+7dagKZ0cgpgrD7Fyho=
 github.com/aler9/go-astits v0.0.0-20210423195926-582b09ed7c04 h1:CXgQLsU4uxWAmsXNOjGLbj0A+0IlRcpZpMgI13fmVwo=
 github.com/aler9/go-astits v0.0.0-20210423195926-582b09ed7c04/go.mod h1:DkOWmBNQpnr9mv24KfZjq4JawCFX1FCqjLVGvO0DygQ=
-github.com/aler9/gortsplib v0.0.0-20210516134729-c6ff1e0d0243 h1:q6xHohYx2qiXE0txZ68hnXg2525mkuG0yG93AXDkV6c=
-github.com/aler9/gortsplib v0.0.0-20210516134729-c6ff1e0d0243/go.mod h1:zVCg+TQX445hh1pC5QgAuuBvvXZMWLY1XYz626dGFqY=
+github.com/aler9/gortsplib v0.0.0-20210516193013-ea6138c28286 h1:j+fO9eKrLwDLpaqB5/d8qOyTS+5u5309XC1ygb7T/Aw=
+github.com/aler9/gortsplib v0.0.0-20210516193013-ea6138c28286/go.mod h1:zVCg+TQX445hh1pC5QgAuuBvvXZMWLY1XYz626dGFqY=
 github.com/aler9/rtmp v0.0.0-20210403095203-3be4a5535927 h1:95mXJ5fUCYpBRdSOnLAQAdJHHKxxxJrVCiaqDi965YQ=
 github.com/aler9/rtmp v0.0.0-20210403095203-3be4a5535927/go.mod h1:vzuE21rowz+lT1NGsWbreIvYulgBpCGnQyeTyFblUHc=
 github.com/asticode/go-astikit v0.20.0 h1:+7N+J4E4lWx2QOkRdOf6DafWJMv6O4RRfgClwQokrH8=