doc: fix incomplete ping session initialization

Thanks @nopnop2002 for reporting this issue.
Closes https://github.com/espressif/esp-idf/issues/4648

docs/en/api-reference/protocols/icmp_echo.rst

@@ -65,6 +65,11 @@ Example method to create a new ping session and register callbacks:
     void initialize_ping()
     {
         /* convert URL to IP address */
+        ip_addr_t target_addr;
+        struct addrinfo hint;
+        struct addrinfo *res = NULL;
+        memset(&hint, 0, sizeof(hint));
+        memset(&target_addr, 0, sizeof(target_addr));
         getaddrinfo("www.espressif.com", NULL, &hint, &res) == 0);
         struct in_addr addr4 = ((struct sockaddr_in *) (res->ai_addr))->sin_addr;
         inet_addr_to_ip4addr(ip_2_ip4(&target_addr), &addr4);