Fix internal error of `->x:@2{}`

[Fix GH-2139] [Bug #15783]

parse.y

@@ -4799,6 +4799,7 @@ f_label 	: tLABEL
 			ID id = get_id($1);
 			arg_var(p, formal_argument(p, id));
 			p->cur_arg = id;
+			p->max_numparam = -1;
 			$$ = $1;
 		    }
 		;

test/ruby/test_syntax.rb

@@ -1301,6 +1301,7 @@ eom
     assert_syntax_error('->(){@1}', /ordinary parameter is defined/)
     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('proc {@1 = nil}', /Can't assign to numbered parameter @1/)
     assert_syntax_error('proc {@01}', /leading zero/)
     assert_syntax_error('proc {@1_}', /unexpected/)