samples: drivers: gpio: Exit from testcase if device not found

Exiting from the test case when the gpio device is not found, this
to avoid a null pointer dereference

This issue was reported by Coverity

Coverity-CID: 151980

Change-Id: I44f13131d44c7c093781e1f11f8481e7ef8175c9
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>

samples/drivers/gpio/src/main.c

@@ -161,6 +161,7 @@ void main(void)
 	gpio_dev = device_get_binding(GPIO_DRV_NAME);
 	if (!gpio_dev) {
 		printk("Cannot find %s!\n", GPIO_DRV_NAME);
+		return;
 	}
 
 	/* Setup GPIO output */