nbd/server: Silence clang sanitizer warning

clang's sanitizer is picky: memset(NULL, x, 0) is technically
undefined behavior, even though no sane implementation of memset()
deferences the NULL.  Caught by the nbd-qemu-allocation iotest.

The alternative to checking before each memset is to instead force an
allocation of 1 element instead of g_new0(type, 0)'s behavior of
returning NULL for a 0-length array.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 3b1f244c (nbd: Allow export of multiple bitmaps for one device)
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20211115223943.626416-1-eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>