fix: blank identifier cause panic (#255)

_test/assign8.go

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	_ = 1
+	println(1)
+}
+
+// Output:
+// 1

interp/cfg.go

@@ -343,7 +343,7 @@ func (interp *Interpreter) cfg(root *node) ([]*node, error) {
 				dest, src := n.child[i], n.child[sbase+i]
 				var sym *symbol
 				var level int
-				if n.kind == defineStmt {
+				if n.kind == defineStmt || (n.kind == assignStmt && dest.ident == "_") {
 					if src.typ != nil && src.typ.cat == nilT {
 						err = src.cfgErrorf("use of untyped nil")
 						break