More SCTP improvements

We're leaking this writable buffer. Since we don't need to hand off ownership, and since SDP_MAX_STRING_LEN is known at build time, this should simply be a stack variable:

char writeable[SDP_MAX_STRING_LEN + 1];
...
sdp_attr_get_sctpmap_protocol(sdp, level, 0, 1, &writable);