Error using metalayers

@FrancescAlted, When I tried to use the metalayers in blosc I get an error.

The example below consists of:
- A schunk based on a frame is created.
- A metalayer is added.
- The schunk is filled using `blosc2_schunk_append_buffer`.

If the metalayers lines are commented it works well. But if the metalayer is added, when it tries to append a buffer, blosc fails.

```C
#include

int main() {

blosc2_frame *frame = blosc2_new_frame("metalayer.blosc2");

blosc2_schunk *sc = blosc2_new_schunk(BLOSC_CPARAMS_DEFAULTS, BLOSC_DPARAMS_DEFAULTS, frame);

char name[] = "metalayer";
uint64_t content = 1234;
uint32_t content_size = sizeof(uint64_t);
blosc2_frame_add_metalayer(sc->frame, name, (uint8_t *) &content, content_size);

uint64_t *content2;
uint32_t content_size2;
blosc2_frame_get_metalayer(sc->frame, name, (uint8_t **) &content2, &content_size2);

printf("Content %llu\n", *content2);

size_t part_size = 100 * 100 * sc->typesize;

uint8_t *part = malloc(part_size);

for (int i = 0; i < 2; ++i) {
int err = blosc2_schunk_append_buffer(sc, part, part_size);
printf("Error %d\n", err);
}

free(part);

return 0;
}
```