This is the last bit fixing <https://bugs.gnu.org/35283>.
Previously, 'mformat' (used by 'grub-mkrescue') would pass uninitialized
bytes to write(2), leading to non-deterministic FAT image contents.
This patch fixes that.
* gnu/packages/patches/mtools-mformat-uninitialized.patch: New file.
* gnu/packages/mtools.scm (mtools)[source]: Use it.
* gnu/local.mk (dist_patch_DATA): Add it.
Ludovic Courtès