driver core: class: Fix wild pointer dereferences in API class_dev_iter_next()

There are a potential wild pointer dereferences issue regarding APIs
class_dev_iter_(init|next|exit)(), as explained by below typical usage:

// All members of @iter are wild pointers.
struct class_dev_iter iter;

// class_dev_iter_init(@iter, @class, ...) checks parameter @class for
// potential class_to_subsys() error, and it returns void type and does
// not initialize its output parameter @iter, so caller can not detect
// the error and continues to invoke class_dev_iter_next(@iter) even if
// @iter still contains wild pointers.
class_dev_iter_init(&iter, ...);

// Dereference these wild pointers in @iter here once suffer the error.
while (dev = class_dev_iter_next(&iter)) { ... };

// Also dereference these wild pointers here.
class_dev_iter_exit(&iter);

Actually, all callers of these APIs have such usage pattern in kernel tree.
Fix by:
- Initialize output parameter @iter by memset() in class_dev_iter_init()
and give callers prompt by pr_crit() for the error.
- Check if @iter is valid in class_dev_iter_next().

Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/r/20250105-class_fix-v6-1-3a2f1768d4d4@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>