Bugfix:Clear pc_tree_stats for all nodes in pctree
Bugfix:Clear pc_tree_stats for all nodes in pctree

As reported in BUG=aomedia:2134, multi-threading
encoding result is mismatch with single-threading.

The mismatch happens on the edge of a frame, some
sub blocks are skipped, so their pc_tree_stats are
not cleared, still contain random values. The
random values in multi-threading case and
single-threading case are different, so the partition
result of ml_prune_2pass_split_partition are different.

This CL try to fix this bug by clearing pc_tree_stats
for all nodes in pc_tree before calling
rd_pick_sqr_partition.

STATS_CHANGED expected

Change-Id: Icb62ff44e262b72fb39f0aa51b82e8e37c7af841

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom