efibootmgr: sanitize `set/get_mirror()`

[rw4s]

get_mirror()

• don't short-circuit the assignment of values in case of version mismatch as show_mirror() happily tries to use them nevertheless
• move redundant warning (from show_mirror()s PoV) to set_mirror()
• add missing free(data)

set_mirror()

• skip obsolete second assignment of data
• avoid potentially "uninitialized" access to above/below4g by protecting it with opts.set_mirror_hi/_lo respectively, thus simplifying the code down the line Note: this will prevent unnecessary write-operations!

[vathpela]

Thanks, I've pushed this to master.