parser: make `v := f(v)' an undefined variable error (#8634)

vlib/v/checker/tests/assign_expr_undefined_err_h.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/assign_expr_undefined_err_h.vv:6:9: error: undefined variable: `n`
+    4 | 
+    5 | fn main() {
+    6 |     n := f(n)
+      |            ^
+    7 |     println(n)
+    8 | }

vlib/v/checker/tests/assign_expr_undefined_err_h.vv

@@ -0,0 +1,8 @@
+fn f(i int) int {
+	return i
+}
+
+fn main() {
+	n := f(n)
+	println(n)
+}

vlib/v/parser/assign.v

@@ -25,6 +25,9 @@ fn (mut p Parser) check_undefined_variables(exprs []ast.Expr, val ast.Expr) ? {
 		}
 		ast.CallExpr {
 			p.check_undefined_variables(exprs, val.left) ?
+			for arg in val.args {
+				p.check_undefined_variables(exprs, arg.expr) ?
+			}
 		}
 		ast.InfixExpr {
 			p.check_undefined_variables(exprs, val.left) ?