libelf: Take map offset into account for Shdr alignment check in elf_begin

libelf: Take map offset into account for Shdr alignment check in elf_begin

The sh_num function tries to get at the zero section Shdr directly.
When the file is mmapped it has to make sure the offset into the file
to the start of the Elf structure is taken into account when trying to
cast the address to make sure the alignment is correct.

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