Fix double-free introduced by commit 33d02a42d64cf06cada1c389e5abba4b9d196cc5

Fix double-free introduced by commit 33d02a42d64cf06cada1c389e5abba4b9d196cc5

To reproduce the problem, make sure you have a GPG public key available, build and install GRUB:
grub-install --debug --debug-image="all" --pubkey=/boot/pubkey.gpg --modules="serial terminfo gzio search search_label search_fs_uuid search_fs_file linux vbe video_fb video mmap relocator verify gcry_rsa gcry_dsa gcry_sha256 hashsum gcry_sha1 mpi echo loadenv boottime" /dev/sda
Sign all the files in /boot/grub/* and reboot.

'make check' results identical before and after this change.

TESTED: In a QEMU VM using an i386 target.