MYNEWT-698: Unhandled exception in ble_ll_conn_end
MYNEWT-698: Unhandled exception in ble_ll_conn_end

This is another issue that should not occur but was discovered
when testing with other devices. If the device sends a TERMINATE
with "unknown connection ID (error 2)" this would cause the
controller to access a NULL variable. This has now been fixed.

X-Original-Commit: a73a65bfce7a86c0ef1d481c2506ab07a19f18fe

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble