lavf/mov: Do not mix variable declaration and code.

Fixes a warning:
libavformat/mov.c:1195:5: warning: ISO C90 forbids mixed declarations and code

libavformat/mov.c

@@ -1188,11 +1188,12 @@ static void set_frag_stream(MOVFragmentIndex *frag_index, int id)
 static MOVFragmentStreamInfo * get_current_frag_stream_info(
     MOVFragmentIndex *frag_index)
 {
+    MOVFragmentIndexItem *item;
     if (frag_index->current < 0 ||
         frag_index->current >= frag_index->nb_items)
         return NULL;
 
-    MOVFragmentIndexItem * item = &frag_index->item[frag_index->current];
+    item = &frag_index->item[frag_index->current];
     if (item->current >= 0 && item->current < item->nb_stream_info)
         return &item->stream_info[item->current];