Fix #1021 - Optional or branch doesn't define err

compiler/tests/option_test.v

@@ -0,0 +1,10 @@
+fn opt_err() ?string {return error('hi')}
+
+fn test_err(){
+	v := opt_err() or {
+		assert err == 'hi'
+		return
+	}
+	assert false
+	println(v) // suppress not used error
+}