In #36 some of the odd errors were due to seeks beyond the end of the
disc. This was because the disc was "specially crafted" to be 25GB + 1
byte, which doesn't fit into the normal 512 byte sectors expected of a
disc. This lead to reads going beyond the end of the disc etc.
If a similarly evil disc is used with `losetup`, it just ignores the
last bytes of the disc that don't fit into 512 chunks. This is what
that patch does, logging an error at the same time.
```
In file included from src/server/bitset.h:4:0,
from src/server/mirror.h:8,
from src/server/flexnbd.h:5,
from src/server/serve.h:8,
from src/server/serve.c:1:
src/server/serve.c: In function 'tryjoin_client_thread':
src/server/serve.c:258:6: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
(uint64_t)status);
^
```
