server: Check strdup allocation failure

Thanks Xiaoke Wang for the report!

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Pavol Žáčik <pzacik@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/server.c

@@ -497,6 +497,11 @@ static size_t callback_receive_banner(const void *data, size_t len, void *user)
             buffer[i] = '\0';
 
             str = strdup(buffer);
+            if (str == NULL) {
+                session->session_state = SSH_SESSION_STATE_ERROR;
+                ssh_set_error_oom(session);
+                return 0;
+            }
             /* number of bytes read */
             processed = i + 1;
             session->clientbanner = str;