block: bs->drv may be NULL in bdrv_debug_resume()

Currently, bdrv_debug_resume() requires every bs->drv in the BDS stack
to be NULL until a bs->drv with an implementation of bdrv_debug_resume()
is found. For a normal function, this would be fine, but this is a
function for debugging purposes and should therefore allow intermediate
BDS not to have a driver (i.e., be "ejected"). Otherwise, it is hard to
debug such situations.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -4055,7 +4055,7 @@ int bdrv_debug_remove_breakpoint(BlockDriverState *bs, const char *tag)
 
 int bdrv_debug_resume(BlockDriverState *bs, const char *tag)
 {
-    while (bs && bs->drv && !bs->drv->bdrv_debug_resume) {
+    while (bs && (!bs->drv || !bs->drv->bdrv_debug_resume)) {
         bs = bs->file;
     }