scsi-generic: check the return value of bdrv_aio_ioctl in execute_command

This fixes the bug introduced by this commit ad54ae80.
The bdrv_aio_ioctl() still could return null and we should return an error
in that case.

Cc: qemu-stable@nongnu.org
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-generic.c

@@ -174,6 +174,9 @@ static int execute_command(BlockDriverState *bdrv,
     r->io_header.flags |= SG_FLAG_DIRECT_IO;
 
     r->req.aiocb = bdrv_aio_ioctl(bdrv, SG_IO, &r->io_header, complete, r);
+    if (r->req.aiocb == NULL) {
+        return -EIO;
+    }
 
     return 0;
 }