Correct the error in the comment for unsafe_yyjson_mut_strncpy

[hxdnshx]

Similar to yyjson_mut_obj_add_strcpy, in fact, the val & len in yyjson_mut_obj_add_strncpy have already been assigned values
and do not need to have their lifetimes guaranteed.

yyjson/src/yyjson.h

Lines 6963 to 6974 in 0eca326

	yyjson_api_inline bool yyjson_mut_obj_add_strncpy(yyjson_mut_doc *doc,
	yyjson_mut_val *obj,
	const char *_key,
	const char *_val,
	size_t _len) {
	if (yyjson_unlikely(!_val)) return false;
	yyjson_mut_obj_add_func({
	val->uni.str = unsafe_yyjson_mut_strncpy(doc, _val, _len);
	if (yyjson_unlikely(!val->uni.str)) return false;
	val->tag = ((uint64_t)_len << YYJSON_TAG_BIT) | YYJSON_TYPE_STR;
	});
	}

The call to unsafe_yyjson_mut_strncpy in the above code also proves this point.

[ibireme]

Thanks.