libdw: Fix memory corruption in libdw_find_split_unit.

libdw: Fix memory corruption in libdw_find_split_unit.

Found by valgrind when trying to match a split unit from a .dwo file
that doesn't contain the split unit (as first) match. We would close
the split Dwarf too early, before we had inspected all units in it.

Add a testcase that simulates this. Which failed (at least under valgrind
as run by make distcheck) before the fix.

Signed-off-by: Mark Wielaard <mark@klomp.org>