ringbuffer: fix rb_get_bytes_used (modulo overflow)

[ljalves]

Using the modulo to wrap around the buffer is unsafe because it's easy to have the values overflowing.

A simple "real" example from my debug: Buffer size is 15Mb (15728640) Pointer values: tail=0x7f4b84c3b841 head=0x7f4b85b267e9

tail - head = -15642536 tail - head + buf_size = 86104 (correct)

(tail - head) % buf_size = 1134680 (wrong)

This "might" fix #3

PS: I just got a DVB-C modulator card (with 4 modulators) and I'm testing it with this tool and will try to enhance it. But I see that this repo isn't updated for quite some time so I'm not sure if you're really interested in PR's...

[nkhs]

How I can find ts stream url?