criu/parasite-syscall.c: init a var

Got the following from clang:

>   CC       criu/parasite-syscall.o
> criu/parasite-syscall.c:1494:6: error: variable 'parasite_size' is used
> uninitialized whenever 'if' condition is false
> [-Werror,-Wsometimes-uninitialized]
>         if (seized_native(ctl))
>             ^~~~~~~~~~~~~~~~~~
> criu/parasite-syscall.c:1503:2: note: uninitialized use occurs here
>         parasite_size += ctl->args_size;
>         ^~~~~~~~~~~~~
> criu/parasite-syscall.c:1494:2: note: remove the 'if' if its condition
> is always true
>         if (seized_native(ctl))
>         ^~~~~~~~~~~~~~~~~~~~~~~

I look at the code and I'm still not quite sure whether parasite_size
can end up being uninitialized. Looks like for CONFIG_X86_64 it can.

So, let's initialize it.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>