ipc_service: backends: icbmsg: Silence maybe-uninitialized warning

If compiling with optimizations (-O2/-Ofast) gcc emits a
maybe-uninitialized warning for `size`.

As far as I can tell, `size` will always be initialized in the cases
where it is used and only left uninitialized in error cases.

Reproduced on main with the multi_endpoint sample on
nrf5340dk/nrf5340/cpuapp.

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>