samples: button: stop the app if device binding fails

Stop the app from running if device binding fails. Otherwise,
dereferencing NULL pointer will happen. This was caught by
Coverity.

Coverity-CID: 151988

Change-Id: I8245d938498a51123249fbd069935900ad660314
Signed-off-by: Baohong Liu <baohong.liu@intel.com>
1 file changed