block: dma_bdrv_* does not return NULL

block: dma_bdrv_* does not return NULL

Initially attempted with the following semantic patch:

@ rule1 @
expression E;
statement S;
@@
  E =
(
   dma_bdrv_io
|  dma_bdrv_read
|  dma_bdrv_write
)
     (...);
(
- if (E == NULL) { ... }
|
- if (E)
    { <... S ...> }
)

which however did not match anything.

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