Bluetooth: Mesh: Fix failing DFU/SR/FD/BV-08-C test

The `upload_status_rsp_with_progress` function uses the currently set
slot pointer to set firmware id field in the message. If another
upload starts but the firmware is already received, the slot API won't
let us to set the fwid and will return an error code. In this case, the
slot pointer stays invalid as fwid wasn't set, and
`upload_status_rsp_with_progress` will not add fwid as the lenght is
zero.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>