[ruby/yarp] Better handle invalid gvar writes

https://github.com/ruby/yarp/commit/eaaebc17c8

test/yarp/errors_test.rb

@@ -1143,6 +1143,12 @@ module YARP
       ]
     end
 
+    def test_invalid_global_variable_write
+      assert_errors expression("$',"), "$',", [
+        ["Immutable variable as a write target", 0..2]
+      ]
+    end
+
     private
 
     def assert_errors(expected, source, errors)

yarp/yarp.c

@@ -8136,12 +8136,9 @@ parse_target(yp_parser_t *parser, yp_node_t *target) {
             target->type = YP_CONSTANT_TARGET_NODE;
             return target;
         case YP_BACK_REFERENCE_READ_NODE:
-            assert(sizeof(yp_global_variable_target_node_t) == sizeof(yp_back_reference_read_node_t));
-            /* fallthrough */
         case YP_NUMBERED_REFERENCE_READ_NODE:
-            assert(sizeof(yp_global_variable_target_node_t) == sizeof(yp_numbered_reference_read_node_t));
             yp_diagnostic_list_append(&parser->error_list, target->location.start, target->location.end, YP_ERR_WRITE_TARGET_READONLY);
-            /* fallthrough */
+            return target;
         case YP_GLOBAL_VARIABLE_READ_NODE:
             assert(sizeof(yp_global_variable_target_node_t) == sizeof(yp_global_variable_read_node_t));
             target->type = YP_GLOBAL_VARIABLE_TARGET_NODE;