lockdown: avoid crashing when getting system buid fails

When userpref_read_system_buid fails to retrieve the buid,
systembuid will be set to NULL. It was then unconditionally passed
to plist_new_string - which will crash with a NULL argument.

See https://bugs.debian.org/730756 for people reporting this
happening in the real world.

See https://github.com/libimobiledevice/libimobiledevice/commit/1331f6baa1799e41003aa812c0d1bf36193535ea
("lockdown: Make sure to set SystemBUID in generated pair records")
for the commit where this problem was introduced.
lockdown: cleanup: remove pointless goto

Minor cleanup. Drop useless code. We're already at "leave",
so there's no need to (conditionally!) goto it.