checker: add error for assigning none values (#13383)

vlib/v/checker/assign.v

@@ -47,6 +47,9 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				c.error('unexpected `mut` on right-hand side of assignment', right.mut_pos)
 			}
 		}
+		if right is ast.None {
+			c.error('you can not assign a `none` value to a variable', right.pos)
+		}
 	}
 	if node.left.len != right_len {
 		if right_first is ast.CallExpr {

vlib/v/checker/tests/assing_none.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assing_none.vv:2:9: error: you can not assign a `none` value to a variable
+    1 | fn main() {
+    2 |     val := none
+      |            ~~~~
+    3 |     println(val)
+    4 | }

vlib/v/checker/tests/assing_none.vv

@@ -0,0 +1,4 @@
+fn main() {
+	val := none
+	println(val)
+}