rtlwifi: silence underflow warning

My static checker complains that we have an upper bound but no lower
bound.  I suspect neither are really required but it doesn't hurt to add
a check for negatives.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtlwifi/core.c

@@ -453,7 +453,8 @@ static void _rtl_add_wowlan_patterns(struct ieee80211_hw *hw,
 	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;