vlm: Error loading /requests/vlm.xml

Variable `loop` was uninitialized and resulted in an error when being
concatenated. Initialize the variable to blank: `local loop = ""`.

Close #6882
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit a75ff33f2b426ffe02994b65a84cf70f178b1aee)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

share/lua/http/requests/vlm.xml

@@ -52,7 +52,8 @@ local function print_table(name,t)
 end
 local function print_media(m)
   local name = m.name
-  local type_, enabled, loop, output
+  local type_, enabled, output
+  local loop = ""
   local inputs = {}
   local options = {}
   local instances = {}