virtio/balloon: Fix a crash when collecting stats

The collect_stats hook dereferences the stats virtio queue without
checking that it has been initialised.

Signed-off-by: Keir Fraser <keirf@google.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220520143706.550169-2-keirf@google.com


Signed-off-by: Will Deacon <will@kernel.org>