libdwl: Add validate_strdata to limit Elf_Data d_size to valid strings.

libdwl: Add validate_strdata to limit Elf_Data d_size to valid strings.

dwfl_module_getsym returns the name of a symbol as found in the
corresponding (symbol) string section. Make sure all names are
correctly zero terminated by making sure the last valid index in a
section/segment Elf_Data contains a zero character.

	* libdwfl/dwfl_module_getdwarf.c (validate_strdata): New
	function taking Elf_Data and restricting d_size to last zero
	char.
	(translate_offs): Call validate_strdata.
	(find_symtab): Likewise for both symstrdata and aux_symstrdata.

https://sourceware.org/bugzilla/show_bug.cgi?id=33099

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