Some fuzzer workarounds

[Mark Wielaard <mark@klomp.org>]

Hi,

I looked over the "ClusterFuzz-External via monorail" emails and found
some "real" issues. But in general it is hard to determined what this
cluster is complaining about. The emails are somewhat opaque and don't
contain proper backtraces (with file and line numbers), nor do they
contain any context on how the target was configured or with which
flags or arguments any fuzzing testcases were run.

The following fixes should fix reading of some broken ar archives and
misaligned access of the section zero Shdr for mmaped ELF files where
the start of the Elf image is at some offset from the start of the
map.

[PATCH 1/2] libelf: Take map offset into account for Shdr alignment
[PATCH 2/2] libelf: Make sure ar_size starts with a digit before

https://code.wildebeest.org/git/user/mjw/elfutils/log/?h=fuzz

I haven't been able to replicate any other issues locally.  I don't
really trust the msan instrumentation, better use valgrind (although
both might be too slow for fuzzing).  There are also some other
misaligned type access checks reported by ubsan, but I don't know if
that is because of ALLOW_UNALIGNED is still defined or not (when
configuring with --enable-analyze-undefined ALLOW_UNALIGNED is not
defined, otherwise it is for some arches, including x86_64).

I don't mind getting rid of ALLOW_UNALIGNED, but it is some work.

Cheers,

Mark