libceph: NULL deref on crush_decode() error path

If there is not enough space then ceph_decode_32_safe() does a goto bad.
We need to return an error code in that situation.  The current code
returns ERR_PTR(0) which is NULL.  The callers are not expecting that
and it results in a NULL dereference.

Fixes: f24e9980eb86 ("ceph: OSD client")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/osdmap.c

@@ -317,6 +317,7 @@ static struct crush_map *crush_decode(void *pbyval, void *end)
 		u32 yes;
 		struct crush_rule *r;
 
+		err = -EINVAL;
 		ceph_decode_32_safe(p, end, yes, bad);
 		if (!yes) {
 			dout("crush_decode NO rule %d off %x %p to %p\n",