Disallow numbered parameter as the default value of optional argument

[Fix GH-2139] [Bug #15783]

parse.y

@@ -4735,6 +4735,7 @@ f_norm_arg	: f_bad_arg
 		| tIDENTIFIER
 		    {
 			formal_argument(p, get_id($1));
+			p->max_numparam = -1;
 			$$ = $1;
 		    }
 		;

test/ruby/test_syntax.rb

@@ -1302,6 +1302,7 @@ eom
     assert_syntax_error('->(x){@1}', /ordinary parameter is defined/)
     assert_syntax_error('->x{@1}', /ordinary parameter is defined/)
     assert_syntax_error('->x:@2{}', /ordinary parameter is defined/)
+    assert_syntax_error('->x=@1{}', /ordinary parameter is defined/)
     assert_syntax_error('proc {@1 = nil}', /Can't assign to numbered parameter @1/)
     assert_syntax_error('proc {@01}', /leading zero/)
     assert_syntax_error('proc {@1_}', /unexpected/)