[SysInfo] Fix inserting USB crash issue

BUG=XWALK-2909

The old implementatin didn't check udev_device_get_sysattr_value(dev, "removable")'s
return value which will be NULL when dev hasn't removable attribute (This will happen
when dev is a partition of a device, not the device itself). When a NULL value
directly as std::stio's parameter, the crash will happen.

This change will check udev_device_get_sysattr_value's return vaule. Only when it's a
valid value, it can continue to do the next thing.
Merge pull request #397 from qjia7/usb_crash

[SysInfo] Fix inserting USB crash issue