wifi: Do not access unitialized timestamp

When the Modified is missing in the keyfile we would just
go on use the unitialized variable.

While we are at it, also drop the unnessary checks for NULL
string before passing it to g_free().

Reported by coverity.

plugins/wifi.c

@@ -960,8 +960,7 @@ static int get_latest_connections(int max_ssids,
 		str = g_key_file_get_string(keyfile,
 					services[i], "Favorite", NULL);
 		if (!str || g_strcmp0(str, "true")) {
-			if (str)
-				g_free(str);
+			g_free(str);
 			g_key_file_free(keyfile);
 			continue;
 		}
@@ -970,19 +969,20 @@ static int get_latest_connections(int max_ssids,
 		str = g_key_file_get_string(keyfile,
 					services[i], "AutoConnect", NULL);
 		if (!str || g_strcmp0(str, "true")) {
-			if (str)
-				g_free(str);
+			g_free(str);
 			g_key_file_free(keyfile);
 			continue;
 		}
 		g_free(str);
 
 		str = g_key_file_get_string(keyfile,
 					services[i], "Modified", NULL);
-		if (str) {
-			g_time_val_from_iso8601(str, &modified);
-			g_free(str);
+		if (!str) {
+			g_key_file_free(keyfile);
+			continue;
 		}
+		g_time_val_from_iso8601(str, &modified);
+		g_free(str);
 
 		ssid = g_key_file_get_string(keyfile,
 					services[i], "SSID", NULL);