dwarf-reader: handle symtab.section_header.sh_entsize == 0

dwarf-reader: handle symtab.section_header.sh_entsize == 0

A broken elf file with a sh_entsize of 0 makes the dwarf reader crash
due to a division by zero. Fix this by validating the input and exiting
early in that case.

	* src/abg-dwarf-reader.cc (load_symbol_maps_from_symtab_section):
	Handle elf file with invalid sh_entsize.
	* tests/test-read-dwarf.cc (test_task::perform): handle empty
	in_abi_path and out_abi_path as 'read only' test.
	(InOutSpec): add test case.
	* tests/data/test-read-dwarf/test25-bogus-binary.elf: new test data.

Signed-off-by: Matthias Maennich <maennich@google.com>