Don't zero out origin of uninitialized bytes in kmsan_internal_set_shadow_origin()

The following test:

```
char buf[4];
kmsan_unpoison_shadow(buf, 1);
kmsan_check_memory(buf, 4);
```

doesn't report any errors.
This could be related to `kmsan_unpoison_shadow()` zeroing out the origin despite there still are uninitialized bytes.
We must only zero out an origin iff the shadow is also zeroed out.