df: fix use of uninitialized variable reported by valgrind

df: fix use of uninitialized variable reported by valgrind

 Conditional jump or move depends on uninitialised value(s)
    at 0x40380C: get_field_values (df.c:840)
    by 0x403E16: get_dev (df.c:994)
    by 0x404D65: get_all_entries (df.c:1364)
    by 0x405926: main (df.c:1714)

* src/df.c (get_dev): Initialize the fsu.fsu_bavail_top_bit_set
member, when adding placeholder entries.
(main): Avoid a "definitely lost" memory leak warning from valgrind,
reported by Bernhard Voelker.