knownnetworks: fix potential out of bounds write

If a very long ssid was used (e.g. CJK characters in SSID), it might do
out of bounds write to static variable for lack of checking the position
before the last snprintf() call.

src/knownnetworks.c

@@ -176,7 +176,8 @@ static const char *known_network_get_path(const struct network_info *network)
 		pos += snprintf(path + pos, sizeof(path) - pos, "%02x",
 				network->ssid[i]);
 
-	snprintf(path + pos, sizeof(path) - pos, "_%s",
+	if (pos < sizeof(path))
+		snprintf(path + pos, sizeof(path) - pos, "_%s",
 			security_to_str(network->type));
 	path[sizeof(path) - 1] = '\0';