cve-update-db-native: avoid incomplete updates

The database update has been done on the original file. In case of
network connection issues, temporary outage of the NVD server or
a similar situation, the function could exit with incomplete data
in the database. This patch solves the issue by performing the update
on a copy of the database. It replaces the main one only if the whole
update was successful.

See https://bugzilla.yoctoproject.org/show_bug.cgi?id=14929



Reported-by: Alberto Pianon <alberto@pianon.eu>
Signed-off-by: Marta Rybczynska <marta.rybczynska@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>