[nrf noup] zephyr: Fix infinite loop in select

fixup! [nrf noup] zephyr: Add support for WPA CLI zephyr

In case of low-memory conditions e.g., in stress tests, the allocation
failure causes the callback to skip recv, there by leaving data on the
socket and causing select to trigger the callback in an infinite loop.

This infinite loop causes threads starvation as WPA supplicant thread
runs at a higher priority, and is manifested as shell freeze.

Fix this by discarding the data on socket in case of memory allocation
failure.

Fixes SHEL-1967.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>

wpa_supplicant/ctrl_iface_zephyr.c

@@ -126,8 +126,20 @@ static void wpa_supplicant_ctrl_iface_receive(int sock, void *eloop_ctx,
 	size_t reply_len = 0;
 
 	buf = os_zalloc(CTRL_IFACE_MAX_LEN + 1);
-	if (!buf)
+	if (!buf) {
+		/* Do a dummy read to drain the data from the socket */
+		static unsigned char dummy[512];
+
+		/* This is expected in OOM conditions, so, do not spam the log */
+		wpa_printf(MSG_DEBUG, "Failed to allocate memory for ctrl_iface receive buffer");
+
+		do {
+			res = recv(sock, dummy, sizeof(dummy),
+				   MSG_TRUNC | MSG_DONTWAIT);
+		} while (res > 0);
 		return;
+	}
+
 	res = recv(sock, buf, CTRL_IFACE_MAX_LEN, 0);
 	if (res < 0) {
 		wpa_printf(MSG_ERROR, "recvfrom(ctrl_iface): %s",