block.c: add assertions to static functions

Following the assertion derived from the API split,
propagate the assertion also in the static functions.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220303151616.325444-18-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>