libdwfl: Don't trust e_shentsize in dwfl_segment_report_module

libdwfl: Don't trust e_shentsize in dwfl_segment_report_module

When calulating the possible section header table end us the actual size
of the section headers (sizeof (Elf32_Shdr) or sizeof (Elf64_Shdr)),
not the ELF header e_shentsize value, which can be corrupted. This
prevents a posssible overflow, but we check the shdrs_end is sane
later anyway.

https://sourceware.org/bugzilla/show_bug.cgi?id=28659

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