parser: check assign_stmt of undefined variable (#8366)

vlib/v/checker/tests/assign_expr_undefined_err_g.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/assign_expr_undefined_err_g.vv:2:14: error: undefined variable: `file`
+    1 | fn main() {
+    2 |     mut file := file.open_file('bees.pdf', 'rw', 0o666)
+      |                 ~~~~
+    3 | }

vlib/v/checker/tests/assign_expr_undefined_err_g.vv

@@ -0,0 +1,3 @@
+fn main() {
+	mut file := file.open_file('bees.pdf', 'rw', 0o666)
+}

vlib/v/parser/assign.v

@@ -23,6 +23,9 @@ fn (mut p Parser) check_undefined_variables(exprs []ast.Expr, val ast.Expr) ? {
 				}
 			}
 		}
+		ast.CallExpr {
+			p.check_undefined_variables(exprs, val.left) ?
+		}
 		ast.InfixExpr {
 			p.check_undefined_variables(exprs, val.left) ?
 			p.check_undefined_variables(exprs, val.right) ?