Fix for segfault when reading /proc/mdstat

Fix for segfault when reading /proc/mdstat

Some kernel versions don't put a space between 'active' and '(auto-read-only)'
in /proc/mdstat.  This causes a parsing problem leaving 'level' set to
NULL which causes a crash.

So synthesise a space there if it is missing, and check for 'level' to
be NULL and don't de-ref if it is.