Avoid int underflow in BLEWriter::write_value()

[codepainters]

This fixes a panic if write_value() is called on a closed connection. See #90.

Not sure if BLE_HS_ENOTCONN is the best error code to use for such a situation, though.

[taks]

The ble_gattc_write_no_rsp_flat function (used after ble_att_mtu) returns BLE_HS_ENOTCONN when unconnected.
Therefore, I belive that BLE_HS_ENOTCONN is the best error code.