[#220] blobstor: Support zero shallow depth of blobovnicza tree

There is a need to support single blobovnicza in blobovnicza tree. This can
be achieved with a width of 1, and a depth of 0 or 1. With depth = 1 one
redundant directory is created, inside which there is a blobovnicza. If the
depth is zero, the blobobnivza will be in the root path. Fix negative
capacity in iterateDeepest method with zero depth.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/blobstor/blobovnicza.go

@@ -463,10 +463,15 @@ func (b *blobovniczas) iterateSortedLeaves(addr *objectSDK.Address, f func(strin
 
 // iterator over directories with blobovniczas sorted by weight.
 func (b *blobovniczas) iterateDeepest(addr *objectSDK.Address, f func(string) (bool, error)) error {
+	depth := b.blzShallowDepth
+	if depth > 0 {
+		depth--
+	}
+
 	_, err := b.iterateSorted(
 		addr,
-		make([]string, 0, b.blzShallowDepth-1),
-		b.blzShallowDepth-1,
+		make([]string, 0, depth),
+		depth,
 		func(p []string) (bool, error) { return f(path.Join(p...)) },
 	)