terminal: return NULL on error in terminal_signal_init
terminal: return NULL on error in terminal_signal_init

Callers expect a NULL on error, and with PR #3171 marking
the pointer as __do_free, we now return a pointer to freed
memory here otherwise.

Signed-off-by: Wolfgang Bumiller <bac7898f539fd233d9ce33be7a9eda13ee647930@proxmox.com>

hallyn/lxc,hallyn/lxc,hallyn/lxc