Handle nil packets when getting LDAPResultCode

[trestletech]

nil packets have been observed when using StartTLS on a connection that
doesn't support it. These previously caused a panic but now will just
result in an unhelpful error.

Mimicking a similar strategy in

ldap/ldap.go

Lines 89 to 93 in 07a7330

	defer func() {
	if r := recover(); r != nil {
	err = NewError(ErrorDebugging, errors.New("ldap: cannot process packet to add descriptions"))
	}
	}()

The nil packets on invalid StartTLS attempts should eventually be
corrected, but this makes the failure much more graceful.

Closes #58

[liggitt]

I'd rather see a nil check on packet, and on response below, than expect a panic

[trestletech]

Updated accordingly, @liggitt

[trestletech]

Hi @liggitt. Not trying to push, but curious if you had an ETA on this change. We're trying to decide if we should vendor our fork temporarily or wait for the merge.

Thanks again.

[liggitt]

sorry for the delay, go ahead and squash to a single commit for merge

[trestletech]

Thanks, @liggitt. Just force-pushed the squashed commit.

[liggitt]

thanks for the contribution