GHSA-m7fp-h3p4-hr49

### Summary The current implementation of `strip_html` can cause an infinite loop when the input string contains `<`, has at least one character before `<`, and no `>` appears after `<`.

### Details The problem is in `src/filters/html.ts`. Specifically, the following part has the infinite loop.

``` // Raw-text blocks (HTML5) plus '<...>' as the catch-all kind; a regex // equivalent is O(n^2) in V8 on unclosed openers. export function strip_html (this: FilterImpl, v: string) { const str = stringify(v) this.context.memoryLimit.use(str.length) const blocks = new Map([['<script', '</script>'], ['<style', '</style>'], ['<!--', '-->'], ['<', '>']]) let out = '' let i = 0 while (i < str.length) { const lt = str.indexOf('<', i) if (lt < 0) return out + str.slice(i) out += str.slice(i, lt) for (const [opener, closer] of blocks) { if (!str.startsWith(opener, lt)) continue const e = str.indexOf(closer, lt + opener.length) if (e >= 0) { i = e + closer.length; break } blocks.delete(opener) } if (i === lt) return out + str.slice(lt) } return out } ```

For the input "a<", the variable `lt` is updated to 1 by `const lt = str.indexOf('<', i)`. However, the variable `i` is never updated from its initial value of 0. This is because in `const e = str.indexOf(closer, lt + opener.length)`, `e` becomes -1, since there is no > after <. Therefore, when execution reaches `if (i === lt) return out + str.slice(lt)`, `i` is 0. This is the same state as at the beginning of the loop. As a result, the same thing is repeated again from that state, causing an infinite loop.

### PoC ``` const { Liquid } = require('liquidjs');

const engine = new Liquid();

engine.parseAndRender('{{ html | strip_html }}', { html: 'a<' }).then(console.log);

console.log("This is never displayed."); ```

### Impact This is an infinite loop vulnerability (cf. https://cwe.mitre.org/data/definitions/835.html). This results in a denial of service (DoS). Although a ReDoS vulnerability has previously been reported in the affected function (cf. https://github.com/harttle/liquidjs/security/advisories/GHSA-r7g9-xpmj-5fcq), this issue can cause a more severe impact than that ReDoS vulnerability with an input of only two characters at minimum.

### Recommended Fix There is an issue with the following conditional branch.

``` if (i === lt) return out + str.slice(lt); ```

The following should fix the issue.

``` if (i <= lt) return out + str.slice(lt); ```