libdw: Handle bogus CU length in dwarf_nextcu.

The length field could be so big that it would wrap around the next_offset.
We don't really care that length is bogus, but we don't want to use it to
calculate the next offset if it is.

Found by afl-fuzz.

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