UPSTREAM: selinux: improve error checking in sel_write_load()

Move our existing input sanity checking to the top of sel_write_load()
and add a check to ensure the buffer size is non-zero.

Move a local variable initialization from the declaration to before it
is used.

Minor style adjustments.

Reported-by: Sam Sun <samsun1006219@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Bug: 386755977
Change-Id: I76ec20258a8ef8a2966e98d523b58a0aa8b49bda
(cherry picked from commit 42c77323)
Signed-off-by: yaozhongmin <yaozhongmin@xiaomi.com>