util/grub-protect: Correct uninit "err" variable

util/grub-protect: Correct uninit "err" variable

In function protect_tpm2_export_tpm2key(), the "err" variable
is uninitialized in the normal (error free) path, so ensure this
defaults to GRUB_ERR_NONE.

This causes the GRUB build to fail with clang (observed with clang-14).

Fixes: 5934bf51c (util/grub-protect: Support NV index mode)
Signed-off-by: Andrew Hamilton <adhamilt@gmail.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>