libdw: Check there is enough space for CU 64bit length, version and type.

We only checked we could read the initial length and after knowing the
version and type whether the unit header was the right size. Also check
there are at least enough bytes to read the 64bit length, version and
unit type bytes.

https://sourceware.org/bugzilla/show_bug.cgi?id=24140



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