Grub hangs if SecureBoot verification fails

[Thomas Frauendorfer]

Dear Team,

During tests I noticed that GRUB 2.04 hangs if SecureBoot verification
through shim_lock fails.

Analyzing the problem I noticed that grub_verifiers_open in
grub-core/commands/verifiers.c unconditionally calls ver->close in its
fail: handling.
Other calls to ver->close in grub_verifiers_open are guarded.

The attached patch makes sure that ver->close is only called when not
NULL and fixed the hang/crash I experienced.

If you have any questions please feel free to contact me.

Kind regards,
Thomas

0001-Only-call-ver-close-if-it-is-set-in-verifier.patch
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