Handle decodeBlock error in blocks.Layer method (#5951)

## Motivation

When an error occurs while decoding a block, it is not handled properly and nil is added to the slice.

sql/blocks/blocks.go

@@ -170,22 +170,25 @@ func GetLayer(db sql.Executor, id types.BlockID) (types.LayerID, error) {
 // Layer returns full body blocks for layer.
 func Layer(db sql.Executor, lid types.LayerID) ([]*types.Block, error) {
 	var (
-		blk *types.Block
-		rst []*types.Block
-		err error
+		blk  *types.Block
+		rst  []*types.Block
+		derr error
 	)
-	if _, err = db.Exec("select id, block from blocks where layer = ?1;", func(stmt *sql.Statement) {
+	if _, err := db.Exec("select id, block from blocks where layer = ?1;", func(stmt *sql.Statement) {
 		stmt.BindInt64(1, int64(lid.Uint32()))
 	}, func(stmt *sql.Statement) bool {
 		id := types.BlockID{}
 		stmt.ColumnBytes(0, id[:])
-		blk, err = decodeBlock(stmt.ColumnReader(1), id)
+		blk, derr = decodeBlock(stmt.ColumnReader(1), id)
+		if derr != nil {
+			return false
+		}
 		rst = append(rst, blk)
 		return true
 	}); err != nil {
 		return nil, fmt.Errorf("select blocks in layer %s: %w", lid, err)
 	}
-	return rst, nil
+	return rst, derr
 }
 
 // IDsInLayer returns list of block ids in the layer.

sql/blocks/blocks_test.go

@@ -313,3 +313,18 @@ func TestLoadBlob(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []int{len(blob1.Bytes), len(blob2.Bytes), -1}, sizes)
 }
+
+func TestLayerForMangledBlock(t *testing.T) {
+	db := sql.InMemory()
+	if _, err := db.Exec("insert into blocks (id, layer, block) values (?1, ?2, ?3);",
+		func(stmt *sql.Statement) {
+			stmt.BindBytes(1, []byte(`mangled-block-id`))
+			stmt.BindInt64(2, 1010101)
+			stmt.BindBytes(3, []byte(`mangled-block`)) // this is actually should encode block
+		}, nil); err != nil {
+		require.NoError(t, err)
+	}
+	rst, err := Layer(db, types.LayerID(1010101))
+	require.Empty(t, rst, 0)
+	require.Error(t, err)
+}