Fix compact proof decoding unaccessed last child trie. (#9715)

* fix no child proof attached but root included.

* small stress test for proof of child tries.

* rust fmt

substrate/primitives/state-machine/Cargo.toml

@@ -35,6 +35,7 @@ tracing = { version = "0.1.22", optional = true }
 hex-literal = "0.3.1"
 sp-runtime = { version = "4.0.0-dev", path = "../runtime" }
 pretty_assertions = "0.6.1"
+rand = { version = "0.7.2", feature = ["small_rng"] }
 
 [features]
 default = ["std"]