Fix NULL pointer access when multi-threading is enabled

This CL fixes NULL pointer access in av1_fill_lr_rates(),
caused by the unit-test 'AVxEncoderThreadAllIntraTestLarge'.
When master thread does not pick any encode job, 'tile_ctx'
is not updated. This was resulting in NULL pointer access.
This CL fixes the issue by passing cm->fc to av1_fill_lr_rates().

BUG=aomedia:3036

Change-Id: I1c0030059772af022a22ce42c213a7be79a9d56d
diff --git a/av1/encoder/pickrst.c b/av1/encoder/pickrst.c
index 2c12cb0..4933be3 100644
--- a/av1/encoder/pickrst.c
+++ b/av1/encoder/pickrst.c
@@ -1708,7 +1708,7 @@
   const int num_planes = av1_num_planes(cm);
   assert(!cm->features.all_lossless);
 
-  av1_fill_lr_rates(&x->mode_costs, x->e_mbd.tile_ctx);
+  av1_fill_lr_rates(&x->mode_costs, cm->fc);
 
   int ntiles[2];
   for (int is_uv = 0; is_uv < 2; ++is_uv)