x86/corruption-check: Fix panic in memory_corruption_check() when boot option without value is provided

memory_corruption_check[{_period|_size}]()'s handlers do not check input
argument before passing it to kstrtoul() or simple_strtoull(). The argument
would be a NULL pointer if each of the kernel parameters, without its
value, is set in command line and thus cause the following panic.

PANIC: early exception 0xe3 IP 10:ffffffff73587c22 error 0 cr2 0x0
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.18-rc8+ #2
[    0.000000] RIP: 0010:kstrtoull+0x2/0x10
...
[    0.000000] Call Trace
[    0.000000]  ? set_corruption_check+0x21/0x49
[    0.000000]  ? do_early_param+0x4d/0x82
[    0.000000]  ? parse_args+0x212/0x330
[    0.000000]  ? rdinit_setup+0x26/0x26
[    0.000000]  ? parse_early_options+0x20/0x23
[    0.000000]  ? rdinit_setup+0x26/0x26
[    0.000000]  ? parse_early_param+0x2d/0x39
[    0.000000]  ? setup_arch+0x2f7/0xbf4
[    0.000000]  ? start_kernel+0x5e/0x4c2
[    0.000000]  ? load_ucode_bsp+0x113/0x12f
[    0.000000]  ? secondary_startup_64+0xa5/0xb0

This patch adds checks to prevent the panic.
Signed-off-by: He Zhe <zhe.he@windriver.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: gregkh@linuxfoundation.org
Cc: kstewart@linuxfoundation.org
Cc: pombredanne@nexb.com
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1534260823-87917-1-git-send-email-zhe.he@windriver.comSigned-off-by: Ingo Molnar <mingo@kernel.org>