Ignore affected rows in UPDATE_DB

We get an error  message from UPDATE_DB , when
the number of affected rows is < 1.

In some cases, its ok to have no affected rows,
like in "update all volumes from all pools", when
there is a pool with no volumes inside.

Fixes #62: update all volumes from all pools gives error when a pool doesnt contain volumes

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>