staging: rtlwifi: silence underflow warning

I'm not totally certain that it's necessary to put an upper limit here.
I think it happens at lower levels. But if we are going to do that then
we should have a lower bound as well.

Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/rtlwifi/core.c

···
 	for (i = 0; i < wow->n_patterns; i++) {
 		memset(&rtl_pattern, 0, sizeof(struct rtl_wow_pattern));
 		memset(mask, 0, MAX_WOL_BIT_MASK_SIZE);
-		if (patterns[i].pattern_len > MAX_WOL_PATTERN_SIZE) {
+		if (patterns[i].pattern_len < 0 ||
+		    patterns[i].pattern_len > MAX_WOL_PATTERN_SIZE) {
 			RT_TRACE(rtlpriv, COMP_POWER, DBG_WARNING,
 				 "Pattern[%d] is too long\n", i);
 			continue;