main: do not exit in case stdin is a closed stream

Weston should not exit if stdin is a closed stream. This allows
to launch with a closed stdin:

  # weston <&-

This fixes screen sharing using weston: Weston closes the stdin
before forking itself to execute the screen sharing instance of
weston. Before this patch screen sharing failed with:
  Screen share failed: No wl_shm found
  unknown child process exited

Fixes: f0d39b22 ("weston: Set CLOEXEC on stdin")
Signed-off-by: Stefan Agner <stefan@agner.ch>