Handle zero sh_entsize in get_soname_of_elf_file

Handle zero sh_entsize in get_soname_of_elf_file

Apparently guile produced ELF files don't set sh_entsize for the
dynamic section. Which would cause a divide by zero. Luckily we do
know how big an dynamic entry should be. So use gelf_fsize for
ELF_T_DYN if sh_entsize is zero.

	  * src/abg-dwarf-reader.cc (get_soname_of_elf_file):
	  Make sure entsize is non-zero before use.

https://sourceware.org/bugzilla/show_bug.cgi?id=29346

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