String.fromCodePoint fails when the input is negative

[sozysozbot]

While String.fromCodePoint is handled in L341-345,

pietron/src/debug.js

Lines 341 to 345 in f8ce860

	try {
	return i > 31 ? String.fromCodePoint(i) : '??';
	} catch (error) {
	return '??';
	}

it is not handled in L249-251.

pietron/src/debug.js

Lines 249 to 251 in f8ce860

	if (lessThan(1)) break;
	output += String.fromCodePoint(stack.pop());
	break;

This causes Pietron to fail:

In my opinion, invalid code points qualify as "operations which cannot be performed", and hence should be ignored.