migration/ram.c: Fix use-after-free in multifd_recv_unfill_packet()

Coverity points out (CID 1400442) that in this code:

    if (packet->pages_alloc > p->pages->allocated) {
        multifd_pages_clear(p->pages);
        multifd_pages_init(packet->pages_alloc);
    }

we free p->pages in multifd_pages_clear() but continue to
use it in the following code. We also leak memory, because
multifd_pages_init() returns the pointer to a new MultiFDPages_t
struct but we are ignoring its return value.

Fix both of these bugs by adding the missing assignment of
the newly created struct to p->pages.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-id: 20190409151830.6024-1-peter.maydell@linaro.org
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>