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>
diff --git a/subsys/ipc/ipc_service/backends/ipc_icbmsg.c b/subsys/ipc/ipc_service/backends/ipc_icbmsg.c
index c7cb5f8..02deaa7 100644
--- a/subsys/ipc/ipc_service/backends/ipc_icbmsg.c
+++ b/subsys/ipc/ipc_service/backends/ipc_icbmsg.c
@@ -461,7 +461,7 @@
 			     int new_size)
 {
 	const struct icbmsg_config *conf = dev_data->conf;
-	size_t size;
+	size_t size = 0;
 	int tx_block_index;
 
 	tx_block_index = buffer_to_index_validate(&conf->tx, buffer, &size);