block: Use g_new0() for a bit of extra type checking

g_new(T, 1) is safer than g_malloc(sizeof(T)), because it returns T *
rather than void *, which lets the compiler catch more type errors.

Missed in commit 02c4f26.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1417697709-13087-1-git-send-email-armbru@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
