interface: fix udev_device_get_sysattr_value return value check

interface: fix udev_device_get_sysattr_value return value check

Reviewing the code I found that return value of function
udev_device_get_sysattr_value() is dereferenced without a check.
udev_device_get_sysattr_value() may return NULL by number of reasons.

v2: VIR_DEBUG added, replaced STREQ(NULLSTR()) with STREQ_NULLABLE()
v3: More checks added, to skip earlier. More verbose VIR_DEBUG.

Signed-off-by: Dmitry Frolov <frolov@swemel.ru>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>