dma: the passed io_func does not return NULL

Initially found with the following semantic patch:

@ type @
BlockDriverAIOCB *x;
expression E;
@@
  x = E;
- if (x == NULL) { ... }

@ acb1 @
expression E, E1;
@@
  E1->acb = E;
- if (E1->acb == NULL) { ... }

@ aiocb1 @
expression E, E1;
@@
  E1->aiocb = E;
- if (E1->aiocb == NULL) { ... }

@ acb @
expression E, E1;
@@
  E1.acb = E;
- if (E1.acb == NULL) { ... }

@ aiocb @
expression E, E1;
@@
  E1.aiocb = E;
- if (E1.aiocb == NULL) { ... }

but changed manually to include an assert.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

dma-helpers.c

@@ -142,9 +142,7 @@ static void dma_bdrv_cb(void *opaque, int ret)
 
     dbs->acb = dbs->io_func(dbs->bs, dbs->sector_num, &dbs->iov,
                             dbs->iov.size / 512, dma_bdrv_cb, dbs);
-    if (!dbs->acb) {
-        dma_complete(dbs, -EIO);
-    }
+    assert(dbs->acb);
 }
 
 static void dma_aio_cancel(BlockDriverAIOCB *acb)