[nrf fromlist] net: lib: coap: Initialize response_truncated

Fix the following compilation warning given when using newlibc:
warning: 'response_truncated' may be used
uninitialized [-Wmaybe-uninitialized]

Issue is not seen with picolibc.

The variable was introduced as part of PR #76257

Upstream PR: zephyrproject-rtos/zephyr#79186

Signed-off-by: Tommi Rantanen <[email protected]>

Author: trantanen

subsys/net/lib/coap/coap_client.c

@@ -894,7 +894,7 @@ static void coap_client_recv(void *coap_cl, void *a, void *b)
 		for (int i = 0; i < num_clients; i++) {
 			if (clients[i]->response_ready) {
 				struct coap_packet response;
-				bool response_truncated;
+				bool response_truncated = false;
 
 				k_mutex_lock(&clients[i]->lock, K_FOREVER);