tools/vm/page_owner_sort.c: fix NULL-pointer dereference when comparing stack traces

tools/vm/page_owner_sort.c: fix NULL-pointer dereference when comparing stack traces

If there is no newline in a block, then strchr returns NULL.  We check for
this in stacktrace_compare, but not when culling.  Fix this (and any
future bugs like it) by replacing NULL stack traces with "" in add_list.

Link: https://lkml.kernel.org/r/20211125162653.1855958-1-seanga2@gmail.com
Fixes: d0abbab9e9e9 ("tools/vm/page_owner_sort.c: sort by stacktrace before culling")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Cc: Changhee Han <ch0.han@lge.com>
Cc: Zhenliang Wei <weizhenliang@huawei.com>
Cc: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Cc: Tang Bin <tangbin@cmss.chinamobile.com>
Cc: Sean Anderson <seanga2@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Yinan Zhang <zhangyinan2019@email.szu.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>