samples: grove_lcd: stop the app if device binding fails

Proceed to LCD programming only if device binding succeeds.
Otherwise, dereferencing a NULL pointer will happen. This
was caught by Coverity.

Coverity-CID: 151986

Change-Id: Ibdb658f530203428aa3e53f358e0788fc1502b06
Signed-off-by: Baohong Liu <baohong.liu@intel.com>

samples/grove/lcd/src/main.c

@@ -55,6 +55,7 @@ void main(void)
 	glcd = device_get_binding(GROVE_LCD_NAME);
 	if (!glcd) {
 		printk("Grove LCD: Device not found.\n");
+		return;
 	}
 
 	/* Now configure the LCD the way we want it */