firmware: stratix10-svc: fix a missing check on list iterator

firmware: stratix10-svc: fix a missing check on list iterator

The bug is here:
	pmem->vaddr = NULL;

The list iterator 'pmem' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case must
be checked before any use of the iterator, otherwise it will
lead to a invalid memory access.

To fix this bug, just gen_pool_free/set NULL/list_del() and return
when found, otherwise list_del HEAD and return;

Fixes: 7ca5ce896524f ("firmware: add Intel Stratix10 service layer driver")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Link: https://lore.kernel.org/r/20220414035609.2239-1-xiam0nd.tong@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>