container: Prevent potential panic of version getter

Previously, `Container.Version` method threw panic when being called on
any instance which had not been protocol-decoded. While this was not a
big problem because this field does not bring any practical benefit, in
general it could generate unsafe code. For example, when the caller does
not know how exactly the accepted instance is created.

Now method returns zero version if underlying protocol field is unset.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

container/container.go

@@ -561,6 +561,8 @@ func (x Container) AssertID(id cid.ID) bool {
 // Version returns the NeoFS API version this container was created with.
 func (x Container) Version() version.Version {
 	var v version.Version
-	_ = v.ReadFromV2(*x.v2.GetVersion()) // No, this can't fail for x.
+	if m := x.v2.GetVersion(); m != nil {
+		_ = v.ReadFromV2(*m)
+	}
 	return v
 }