Don't fail findUpMedia when serverConnection is null.

This avoids throwing if the user changes their setting before
login.  Thanks to Jean-Jacques Sarton.

static/galene.js

@@ -1761,6 +1761,8 @@ function closeUpMedia(label) {
  * @returns {Stream}
  */
 function findUpMedia(label) {
+    if(!serverConnection)
+        return null;
     for(let id in serverConnection.up) {
         let c = serverConnection.up[id];
         if(c.label === label)