Fixes for memory access violations triggered by running addr2line on fuzzed binaries.

Fixes for memory access violations triggered by running addr2line on fuzzed binaries.

	PR binutils/17512
	* addr2line.c (slurp_symtab): If the symcount is zero, free the
	symbol table pointer.

	* dwarf2.c (concat_filename): Check for an empty directory table.
	(scan_unit_for_symbols): Check for reading off the end of the
	unit.
	(parse_comp_unit): Check for a DW_AT_comp_dir attribute with a
	non-string form.
	* elf64-ppc.c (opd_entry_value): Fail if there are no relocs
	available.