monitor: Avoid segfault when calling NULL get_bad_blocks

monitor: Avoid segfault when calling NULL get_bad_blocks

Not all struct superswitch implement a get_bad_blocks() function,
yet mdmon seems to call it without checking for NULL and thus
occasionally segfaults in the test 10ddf-geometry.

Fix this by checking for NULL before calling it.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>