Fix "ip route" parsing

If a line did not end with '\r', then the final `\n' was replaced by
'\0' for parsing the current line. This `\0` was then mistakenly
considered as the end of the whole "ip route" output, so the remaining
lines were not parsed, causing "scrcpy --tcpip" to fail in some cases.

To fix the issue, read the final character of the current line before it
is (possibly) overwritten by '\0'.