Staging: rtl8188eu: core: rtw_mlme.c: Replace NULL comparision.

Replace NULL comparison with '!' in the file rtw_mlme.c
Issue found by checkpatch.pl semantic patch results for rtw_mlme.c

CHECK:Comparision to NULL could be written "!candidate"
Signed-off-by: Sanjana Sanikommu <sanjana99reddy99@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/core/rtw_mlme.c

@@ -1438,7 +1438,7 @@ static int rtw_check_join_candidate(struct mlme_priv *pmlmepriv
 			goto exit;
 	}
 
-	if (*candidate == NULL || (*candidate)->network.Rssi < competitor->network.Rssi) {
+	if (!*candidate || (*candidate)->network.Rssi < competitor->network.Rssi) {
 		*candidate = competitor;
 		updated = true;
 	}