test/dm: check if devices exist

Running 'ut dm' on the sandbox without -D or -d results in segmentation
faults due to NULL pointer dereferences.

Check that device pointers are non-NULL before using them.

Use ut_assertnonnull() for pointers instead of ut_assert().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Philippe Reynes <philippe.reynes@softathome.com>