BZ#331380 Syscall param timer_create(evp) points to uninitialised byte(s)

Only check struct sigevent actually used by the kernel. If SIGEV_THREAD_ID
is set check sigev_notify_thread_id, otherwise don't.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13835 a5019735-40e9-0310-863c-91ae7b9d1cf9