sandbox/seccomp: fake close(2) return value

Some errors will cause the XMPP file descriptor to be closed before
xmppipe exits. Return EBADF if close is called since the process will
terminate anyway.

src/xmppipe_sandbox_seccomp.c

@@ -90,6 +90,9 @@ xmppipe_sandbox_init(xmppipe_state_t *state)
 #ifdef __NR_open
         SC_DENY(open, EACCES),
 #endif
+#ifdef __NR_close
+        SC_DENY(close, EBADF),
+#endif
 
         /* Syscalls to allow */
 #ifdef __NR_brk