ipc-unix: leave room for a NUL terminator

The `strncpy` function will not always place a terminating `NUL` if the
source is longer than the destination buffer. Instead, let `strncpy`
truncate as it normally would, but leave the last byte alone (it is
zero-initialized when declared).

input/ipc-unix.c

@@ -324,7 +324,7 @@ static void *ipc_thread(void *p)
     }
 
     ipc_un.sun_family = AF_UNIX,
-    strncpy(ipc_un.sun_path, arg->path, sizeof(ipc_un.sun_path));
+    strncpy(ipc_un.sun_path, arg->path, sizeof(ipc_un.sun_path) - 1);
 
     unlink(ipc_un.sun_path);