Filip Roséen authored Feb 25, 2016 and Jean-Baptiste Kempf committed Feb 26, 2016

If the remote server yields an error on the `DESCRIBE` request, while
also including an error message the module would crash due to an invalid
free.

% netcat -l -p 8080 <<EOF
> RTSP/1.0 200 OK
> CSeq: 1
> Server: Real
> RealChallenge1: DEADBEEF
>
> RTSP/1.0 199 OK
> CSeq: 2
> Alert: I like turtles
>
EOF

The reason being that `alert` will point to the middle of allocated
memory. Given the sourroundings the original author probably forgot to
`strdup` the message - even though that is very unnecessary.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>