libelf, readelf: Use validate_str also to check dynamic symstr data

libelf, readelf: Use validate_str also to check dynamic symstr data

When dynsym/str was read through eu-readelf --dynamic by readelf
process_symtab the string data was not validated, possibly printing
unallocated memory past the end of the symstr data. Fix this by
turning the elf_strptr validate_str function into a generic
lib/system.h helper function and use it in readelf to validate the
strings before use.

	* libelf/elf_strptr.c (validate_str): Remove to...
	* lib/system.h (validate_str): ... here. Make inline, simplify
	check and document.
	* src/readelf.c (process_symtab): Use validate_str on symstr_data.

https://sourceware.org/bugzilla/show_bug.cgi?id=32654

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