ntdll: Zero fl.l_pid field in user space before reading from it.

[Jefferson Carpenter]

When this code path is hit, memory sanitizers such as valgrind
currently report an uninitialized read at fl.l_pid.  This is most
likely because the value is set in kernel space, which is
out-of-bounds to user-space programs.  This patch clears up that
output, and ensures that there will not be a memory vulnerability even
in the event that the kernel does not set l_pid (notwithstanding that
POSIX specifies it should).

- Jefferson