acpi: verify file entries in bios_linker_loader_add_pointer()

acpi: verify file entries in bios_linker_loader_add_pointer()

The callers to bios_linker_find_file() assert that the file entry returned
is not NULL, except for those in bios_linker_loader_add_pointer().  Add two
asserts in that case for completeness and to facilitate static code analysis.

Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <1553199229-25318-1-git-send-email-liam.merwick@oracle.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>