util/glue-efi: Fix incorrect use of a possibly negative value

util/glue-efi: Fix incorrect use of a possibly negative value

It is possible for the ftell() function to return a negative value,
although it is fairly unlikely here, we should be checking for
a negative value before we assign it to an unsigned value.

Fixes: CID 73744
Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Patch-Name: 2021-02-security/057-util-glue-efi-Fix-incorrect-use-of-a-possibly-negative-value.patch