fix(rtc): Bound the text a description's feedback prints back
An a=rtcp-fb line is kept, and printed, once for each codec it lands
on, every codec of its section for *. MAX_RTCP_FB bounds the entries
a section holds but not their length, so a few long wildcard lines
printed back many times what was read: the sdp fuzz target's case
(OSS-Fuzz issue 585919670), two 20,838-character * lines over 15
codecs, 67,619 characters printed as 663,517, past the twice
MAX_REMOTE_SDP the target asserts since fb9a5ed.
Each line now adds its value's length plus 16 (what the line around it
prints), times the codecs it lands on, to a count across the
description, and a description past MAX_RTCP_FB_TEXT (64 KiB) is
refused, as one past the other bounds is. A browser's offer prints a
few kilobytes of feedback.
The case aborts at the target's bound on main and runs clean with
this. A test refuses its shape and holds the bound to the byte (it
fails with the check removed), and the description at every bound now
carries a section at both feedback bounds and still prints back within
twice the read limit.