Handle null and int parse errors in frame function arg values

[stephen-f0]

nullptr args now handle / display correctly

__libc_start_main_impl(main=0x555555555149 argc=0x1 argv=0x7fffffffe038 init=null fini=null rtld_fini=null stack_end=0x7fffffffe028)

Also added extra checks around the arg value.GetValue() call when casting to an int to safely handle ValueError exceptions