fix: do not panic when assigning to _ (blank) var.

Fix interp.isEmptyInterface to tolerate a nil type.

Fixes #1619.

.gitignore

@@ -1,6 +1,7 @@
 .*.swo
 .*.swp
 *.dot
+*.out
 .idea/
 /yaegi
 internal/cmd/extract/extract

_test/op10.go

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

_test/op11.go

@@ -0,0 +1,10 @@
+package main
+
+func main() {
+	a, b := 1, 2
+	_ = a + b
+	println("ok")
+}
+
+// Output:
+// ok

interp/type.go

@@ -2393,7 +2393,7 @@ func isMap(t *itype) bool  { return t.TypeOf().Kind() == reflect.Map }
 func isPtr(t *itype) bool  { return t.TypeOf().Kind() == reflect.Ptr }
 
 func isEmptyInterface(t *itype) bool {
-	return t.cat == interfaceT && len(t.field) == 0
+	return t != nil && t.cat == interfaceT && len(t.field) == 0
 }
 
 func isGeneric(t *itype) bool {