du: avoid abort on systems for which ->me_type is not malloc'd

du: avoid abort on systems for which ->me_type is not malloc'd

On some systems (notably, BSD-based, like at least OpenBSD 4.9),
the me_type member does not come from the heap.
* src/du.c (fill_mount_table): Free the ->me_type member only
when it was malloc'd, i.e., when ->me_type_malloced is nonzero.
Bug introduced via commit v8.19-2-gcf7e1b5.
Reported as http://bugs.gnu.org/12542.