vp9_loop_filter_frame_mt: defer allocations

the code currently checks whether the allocation has been done instead
of allocating on the first frame.
since:
4f27202 vp9: fix crash in mt loopfilter w/corrupt file

this change defers the allocation until the loop filter is used.

Change-Id: I660c1b7f34e713a8dd9884483f01d23b9847366e

vp9/decoder/vp9_dthread.c

diff --git a/vp9/decoder/vp9_dthread.c b/vp9/decoder/vp9_dthread.c
index e6ce14d..1572aa4 100644
--- a/vp9/decoder/vp9_dthread.c
+++ b/vp9/decoder/vp9_dthread.c
@@ -145,15 +145,13 @@
   const int num_workers = MIN(pbi->max_threads & ~1, tile_cols);
   int i;
 
-  // Allocate memory used in thread synchronization.
-  // This always needs to be done even if frame_filter_level is 0.
+  if (!frame_filter_level) return;
+
   if (!lf_sync->sync_range || cm->last_height != cm->height) {
     vp9_loop_filter_dealloc(lf_sync);
     vp9_loop_filter_alloc(lf_sync, cm, sb_rows, cm->width);
   }
 
-  if (!frame_filter_level) return;
-
   vp9_loop_filter_frame_init(cm, frame_filter_level);
 
   // Initialize cur_sb_col to -1 for all SB rows.