CB_HANDLER not ensuring that callback exists for helpers

[reidspreiter]

When a helper is called by CB_HANDLER, the callback function is not checked for NULL, but it is checked before being called directly:

#define CB_HANDLER(name)						       \
	case CB_TYPE_##name :						       \
	{								       \
		const struct name ## _callback *cb = &callback->callback.name ;\
		if (cb && cb->helper) {					       \
			CKINT(cb->helper(processdata, parsed_flags, testvector,\
					 testresults, cb->fn, cb->vector));    \
		} else if (cb && cb->fn) {				       \
			CKINT(cb->fn(cb->vector, parsed_flags));	

I set some ecdsa_backend callbacks to NULL because I don't build the openssl code used by the callbacks. This caused a crash when testing vectors instead of returning EOPNOTSUPP.

Is this expected behavior? If not, I attached a simple patch I used to fix this on my end.

parser_common.patch

Thanks,
Reid