youtube.lua: factor out descrambling javascript fetching

We'll be descrambling the "n" parameter in addition to the URL signature
using this same javascript web asset, so we want to be able to share and
reuse it.

share/lua/playlist/youtube.lua

@@ -113,14 +113,8 @@ end
 
 -- Descramble the URL signature using the javascript code that does that
 -- in the web page
-function sig_descramble( sig, js_url )
-    if not js_url then
-        return nil
-    end
-
-    -- Fetch javascript code
-    local js = { stream = vlc.stream( js_url ), lines = {}, i = 0 }
-    if not js.stream then
+function sig_descramble( sig, js )
+    if not js then
         return nil
     end
 
@@ -207,7 +201,7 @@ function sig_descramble( sig, js_url )
 end
 
 -- Parse and assemble video stream URL
-function stream_url( params, js_url )
+function stream_url( params, js )
     local url = string.match( params, "url=([^&]+)" )
     if not url then
         return nil
@@ -219,7 +213,7 @@ function stream_url( params, js_url )
     if s then
         s = vlc.strings.decode_uri( s )
         vlc.msg.dbg( "Found "..string.len( s ).."-character scrambled signature for youtube video URL, attempting to descramble... " )
-        local ds = sig_descramble( s, js_url )
+        local ds = sig_descramble( s, js )
         if not ds then
             vlc.msg.dbg( "Couldn't descramble YouTube video URL signature" )
             vlc.msg.err( "Couldn't process youtube video URL, please check for updates to this script" )
@@ -288,13 +282,23 @@ function pick_stream( stream_map, js_url )
         return nil
     end
 
+    -- Fetch javascript code: we'll need this to descramble maybe the
+    -- URL signature, and normally always the "n" throttling parameter.
+    local js = nil
+    if js_url then
+        js = { stream = vlc.stream( js_url ), lines = {}, i = 0 }
+        if not js.stream then
+            js = nil
+        end
+    end
+
     -- Either the "url" or the "signatureCipher" parameter is present,
     -- depending on whether the URL signature is scrambled.
     local cipher = string.match( pick, '"signatureCipher":"(.-)"' )
         or string.match( pick, '"[a-zA-Z]*[Cc]ipher":"(.-)"' )
     if cipher then
         -- Scrambled signature: some assembly required
-        local url = stream_url( cipher, js_url )
+        local url = stream_url( cipher, js )
         if url then
             return url
         end