Patch for zmkfirmware#1243: avoid write to uninitialized handle

In split_central_update_indicators_callback, it seems that the peripheral is
sometimes considered connected before the GATT characteristics have been
discovered. If this is the case, the update_hid_indicators handle will not yet
be set, and must not be written to.

Author: chrisandreae

app/src/split/bluetooth/central.c

@@ -713,6 +713,13 @@ static void split_central_update_indicators_callback(struct k_work *work) {
             continue;
         }
 
+        if (peripherals[i].update_hid_indicators == 0) {
+            // It appears that sometimes the peripheral is considered connected
+            // before the GATT characteristics have been discovered. If this is
+            // the case, the update_hid_indicators handle will not yet be set.
+            continue;
+        }
+
         int err = bt_gatt_write_without_response(peripherals[i].conn,
                                                  peripherals[i].update_hid_indicators, &indicators,
                                                  sizeof(indicators), true);