That parameter assumed that the query was part of the path. However it
is not, which leads to invalid host name, and eventually failure when
the path is missing. In practice, passing any value other than '?' as
separator would not work properly.

Remaining vlc_UrlParse() call sites without the option separator do not
support query at the protocol level anyway, so they are unaffected by the
change.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>