save_env prepends \0 to grubenv, causing "error: invalid environment block."

[Clara Short]

Reported upstream as https://savannah.gnu.org/bugs/index.php?58283 .

= Configuration =
grub> echo $package_version $grub_cpu $grub_platform
2.02-2ubuntu8.15 i386 pc

== 1. Initial conditions ==
# rm /boot/grub/grubenv
# grub-editenv - create
# grub-editenv - set foo=bar
# grub-editenv - list
foo=bar
# hexdump -C /boot/grub/grubenv
00000000 23 20 47 52 55 42 20 45 6e 76 69 72 6f 6e 6d 65 |# GRUB Environme|
00000010 6e 74 20 42 6c 6f 63 6b 0a 66 6f 6f 3d 62 61 72 |nt Block.foo=bar|
00000020 0a 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 |.###############|
00000030 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 |################|
*
00000400
# reboot

== 2. Drop to GRUB shell ==
grub> list_env
foo=bar
grub> baz=quux
grub> save_env baz
grub> list_env
error: invalid environment block.
grub> reboot

== 3. Boot into Linux ==
$ grub-editenv - list
grub-editenv: error: invalid environment block.
$ hexdump -C /boot/grub/grubenv
00000000 00 23 20 47 52 55 42 20 45 6e 76 69 72 6f 6e 6d |.# GRUB Environm|
00000010 65 6e 74 20 42 6c 6f 63 6b 0a 66 6f 6f 3d 62 61 |ent Block.foo=ba|
00000020 72 0a 62 61 7a 3d 71 75 75 78 0a 23 23 23 23 23 |r.baz=quux.#####|
00000030 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 |################|
*
00000400
$

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: grub-common 2.02-2ubuntu8.15
ProcVersionSignature: Ubuntu 4.15.0-99.100-generic 4.15.18
Uname: Linux 4.15.0-99-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
CurrentDesktop: XFCE
Date: Fri May 1 10:35:28 2020
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)