memory allocation error

in jwe.c, on line 1952:
```
cek = cjose_get_alloc()(cek_len);
memcpy(cek, jwe->cek, cek_len);
```

Allocation result is not checked. We should add
```
if (!cek) {
   CJOSE_ERROR(err, CJOSE_ERR_NO_MEMORY);
   return NULL;
}
```