rtp: fix non null terminated string / non portable sscanf

rtp: fix non null terminated string / non portable sscanf

In rtp.c:

if (sscanf(t+9, "%i %64c", &_payload, c) == 2)

the string c seems to be non-null terminated. It is later used as
following:

c[strcspn(c, "\n")] = 0;

The same piece of code is responsible for the inability of pulseaudio
on OpenWRT to handle RTP stream at the rate 48000 from another
machine:

[pulseaudio] sdp.c: Failed to parse SDP data: missing data.

It turns out that uClibc does not agree with glibc about "%64c", see
http://git.uclibc.org/uClibc/tree/docs/Glibc_vs_uClibc_Differences.txt

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=92568