libelf: Check ELF parent size can contain ar member

libelf: Check ELF parent size can contain ar member

Don't trust the ar header offset and size. When creating an Elf
descriptor for an ar member check the offset isn't past the end of the
containing Elf and don't use/set the member maximum_size larger than
the remaining size of the parent.

	* libelf/elf_begin.c (dup_elf): Only call read_file if the
        offset isn't past the end and with a maximum_size not too large.

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