This commit addresses a panic that occurs in older versions of MacOS
(tested with 10.11.5).  The problem occurs because, on this machine, the
DiscoverDescriptors response lacks a `kCBMsgArgResult` field.  This
missing field must have gotten fixed in a subsequent version of MacOS.
Here are the contents of the DiscoverDescriptors response:

```
map[kCBMsgArgCharacteristicHandle:17 kCBMsgArgDeviceUUID:3e8e65b72da94f70bbbe2d8f1a2524ee kCBMsgArgDescriptors:[]]
```

The fix is to treat a response that lacks the `kCBMsgArgResult` field as
indicating success.  That is, pretend `kCBMsgArgResult` is 0.