Remove trailing zero-size arrays from JSString

[bnoordhuis]

It's been reported that UBSan's -fsanitize=bounds-strict does not like empty arrays. Remove them and replace their uses with old school pointer arithmetic.

Fixes: #928

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I had to remove function argument const-ness in a number of places. It was either that or introduce additional str8_c and str16_c functions, but that's both more work and more to remember.

If you think it's a good approach, I'll run benchmarks to see if it doesn't regress performance. (It shouldn't, but hey, compilers.)

[saghul]

LGTM!

[bnoordhuis]

For posterity, the build error is a false positive (buf is not uninitialized) but I don't understand why it would pop up now all of a sudden. Compilers, man.

Details

 In function ‘js_free_rt’,
    inlined from ‘js_free’ at /home/runner/work/quickjs/quickjs/quickjs.c:1521:5,
    inlined from ‘js_string_normalize’ at /home/runner/work/quickjs/quickjs/quickjs.c:42717:13:
/home/runner/work/quickjs/quickjs/quickjs.c:1427:23: error: ‘buf’ may be used uninitialized [-Werror=maybe-uninitialized]
 1427 |     s->malloc_size -= rt->mf.js_malloc_usable_size(ptr) + MALLOC_OVERHEAD;
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/runner/work/quickjs/quickjs/quickjs.c: In function ‘js_string_normalize’:
/home/runner/work/quickjs/quickjs/quickjs.c:42683:15: note: ‘buf’ was declared here
42683 |     uint32_t *buf, *out_buf;