Revert "Remove useless assignation"

This reverts commit 33835542.
It segfaults as p_next is now uninitialized.

==25869== Conditional jump or move depends on uninitialised value(s)
==25869==    at 0x5078332: GetNextItem (tree.c:441)
==25869==    by 0x5078D7A: playlist_GetNextLeaf (tree.c:362)
==25869==    by 0x5071CFE: ResetCurrentlyPlaying (thread.c:197)
==25869==    by 0x5072952: Thread (thread.c:596)
==25869==    by 0x577AFC6: start_thread (in /lib/libpthread-2.7.so)
==25869==    by 0x5EE65AC: clone (in /lib/libc-2.7.so)

src/playlist/thread.c

@@ -191,9 +191,9 @@ static void ResetCurrentlyPlaying( playlist_t *p_playlist,
               PLI_NAME( p_sys->status.p_node ) );
     ARRAY_RESET( p_playlist->current );
     p_playlist->i_current_index = -1;
-    for( ;; )
-    {   /** FIXME: this is *slow* */
-        playlist_item_t *p_next;
+    for( playlist_item_t *p_next = NULL; ; )
+    {
+        /** FIXME: this is *slow* */
         p_next = playlist_GetNextLeaf( p_playlist,
                                        p_sys->status.p_node,
                                        p_next, true, false );