efi_loader: avoid out of bound access in efi_get_variable()

In efi_get_variable() a string is longer than the allocated space which
results in overwriting the linked list of malloc().

The prefixes used for variables are 41 characters long, e.g.
efi_67029eb5-0af2-f6b1-da53-fcb566dd1ce6_

Change PREFIX_LEN to 41.

Fixes: faff21556748 ("efi_loader: remove limit on variable length")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>