init_allowed_partitions: fix logic for Uneven 4-way

Old code was working by accident, due to other logic downstream.

But we correct the logic for safety.

No change in stats.
diff --git a/av1/encoder/encodeframe_utils.h b/av1/encoder/encodeframe_utils.h
index 45bf7a4..c714e9c 100644
--- a/av1/encoder/encodeframe_utils.h
+++ b/av1/encoder/encodeframe_utils.h
@@ -203,7 +203,6 @@
 #if CONFIG_ML_PART_SPLIT
   bool prune_partition_split;
 #endif  // CONFIG_ML_PART_SPLIT
-  bool ext_partition_allowed;
   bool partition_3_allowed[NUM_RECT_PARTS];
   bool prune_partition_3[NUM_RECT_PARTS];
   bool partition_4a_allowed[NUM_RECT_PARTS];
diff --git a/av1/encoder/partition_search.c b/av1/encoder/partition_search.c
index 36ec2b7..67faefc 100644
--- a/av1/encoder/partition_search.c
+++ b/av1/encoder/partition_search.c
@@ -3851,7 +3851,7 @@
       is_bsize_geq(vert_subsize, blk_params->min_partition_size) &&
       is_vert_size_valid;
 
-  const int ext_partition_allowed = part_search_state->ext_partition_allowed =
+  const int ext_partition_allowed =
       part_search_state->is_block_splittable &&
       part_cfg->enable_ext_partitions &&
       is_ext_partition_allowed_at_bsize(bsize, tree_type);
@@ -3887,9 +3887,9 @@
       ;
 
   const int uneven_4way_partition_allowed =
-      part_search_state->ext_partition_allowed =
-          part_cfg->enable_ext_partitions &&
-          is_uneven_4way_partition_allowed_at_bsize(bsize, tree_type);
+      part_search_state->is_block_splittable &&
+      part_cfg->enable_ext_partitions &&
+      is_uneven_4way_partition_allowed_at_bsize(bsize, tree_type);
   part_search_state->partition_4a_allowed[HORZ] =
       uneven_4way_partition_allowed &&
       get_partition_subsize(bsize, PARTITION_HORZ_4A) != BLOCK_INVALID &&