ipc: exit client if the FD is invalid

This does not normally happen. But since the --input-ipc-client option
can pass in raw FDs, it's probably a good thing in the interest of
making mistakes obvious. Without this, it just burned a core on invalid
FDs (poll() always returned immediately).

input/ipc-unix.c

@@ -163,7 +163,7 @@ static void *client_thread(void *p)
             }
         }
 
-        if (fds[1].revents & (POLLIN | POLLHUP)) {
+        if (fds[1].revents & (POLLIN | POLLHUP | POLLNVAL)) {
             while (1) {
                 char buf[128];
                 bstr append = { buf, 0 };