Fix the SEGV exception

xyz.openbmc_project.Dump.Manager.service: Main process exited, code=dumped, status=11/SEGV
xyz.openbmc_project.Dump.Manager.service: Failed with result 'core-dump'.

During the process of deleting all dump entries,
the operation on the iterator may lead to segv fault
because the content that the iterator is pointing to has been freed.

We can move the iterator to the next before deleting the entry.

Signed-off-by: Stanley Chu <yschu@nuvoton.com>
Change-Id: I973e3d3f4df9dc52ee40423553c395e32d3b8d31