Address integer overflow reported by a code analyser

The output of /usr/sbin/netstat would have to be larger than
2 GB for this to be a menace. Because this seems quite improbable,
address with an assert() for now.

src/ipv4.c

@@ -232,6 +232,7 @@ static int ipv4_get_route(struct rtentry *route)
 	while (fgets(line, buffer_size - total_bytes_read - 1, fp) != NULL) {
 		uint32_t bytes_read = strlen(line);
 
+		assert(total_bytes_read < UINT32_MAX - bytes_read);
 		total_bytes_read += bytes_read;
 
 		if (bytes_read > 0 && line[bytes_read - 1] != '\n') {