fix(gnovm): load map from state

Fix gnolang#817

There was actually 2 bugs in the code responsible for loading a map from
the state:
- `UnmarshalAmino` doesn't check that `ml.Tail` is nil before accessing
  it. It is correctly done for instance in `MapList.Append` for
  instance. The fix just ensures that `ml.Tail` is not nil.
- `fillTypesOfValue` which is called just after the store data is
  unmarshalSized, resets the map data by calling `MapValue.MakeMap()`.
  So at the end the map is always empty, whatever was stored before the
  transaction. The fix replaces the call of `MakeMap` to a simple map
  creation.

This has to be confirmed, but this change may also fix gnolang#311, because I'm
not convinced that the indetermenistic behavior of the golang map can be
a problem when saved to the store. Indeed, a gno map is serialized with
the help of a slice, which should remove any indetermenistic problem.

gnovm/pkg/gnolang/realm.go

@@ -1341,7 +1341,7 @@ func fillTypesOfValue(store Store, val Value) Value {
 		fillTypesTV(store, &cv.Receiver)
 		return cv
 	case *MapValue:
-		cv.MakeMap(cv.List.Size) // TODO move out.
+		cv.vmap = make(map[MapKey]*MapListItem, cv.List.Size)
 		for cur := cv.List.Head; cur != nil; cur = cur.Next {
 			fillTypesTV(store, &cur.Key)
 			fillTypesTV(store, &cur.Value)

gnovm/pkg/gnolang/values.go

@@ -11,6 +11,7 @@ import (
 	"unsafe"
 
 	"github.com/cockroachdb/apd"
+
 	"github.com/gnolang/gno/tm2/pkg/crypto"
 )
 
@@ -661,7 +662,9 @@ func (ml *MapList) UnmarshalAmino(mlimg MapListImage) error {
 			ml.Head = item
 		}
 		item.Prev = ml.Tail
-		ml.Tail.Next = item
+		if ml.Tail != nil {
+			ml.Tail.Next = item
+		}
 		ml.Tail = item
 		ml.Size++
 	}

gnovm/tests/challenges/persist_map.gno

@@ -11,8 +11,12 @@ func init() {
 
 func main() {
 	println("premain", amap)
-	amap["b"] = "2"
+	amap["c"] = "3"
 	println("postmain", amap)
 }
 
-// Current behavior: panics
+// Output:
+// preinit map{("a" string):("1" string)}
+// postinit map{("a" string):("1" string),("b" string):("2" string)}
+// premain map{("a" string):("1" string),("b" string):("2" string)}
+// postmain map{("a" string):("1" string),("b" string):("2" string),("c" string):("3" string)}