Added connection Handle check before gap event is handled in characteristic to fix the deadlock issue in #151

[vermasrijan19]

The handle_gap_event that lead to deadlocks where an invalid connection handle 65535 would occur after a failed notify ( no clue what caused that to occur).
The connection handle value was 65535(highest value for u16) which is an invalid conn_handle (unless someone somehow managed to connect that many devices).

The try_lock() worked as a stop gap but it would lead to failed read/write attempts when notify was in progress which would lead to disconnection.

[taks]

Thanks @vermasrijan19 .
Colud you change to use early return as follows.

if crate::utilities::ble_gap_conn_find(conn_handle).is_err() {
  ::log::warn!("the conn handle does not exist");
  return sys::BLE_ATT_ERR_UNLIKELY as _;
}