sdp: harden SDPFragment.Unmarshal against out-of-range accesses

[boks1971]

SDPFragment.Unmarshal indexed into a line knowing only that it was non-empty:

• line[2:] on an m line — a bare "m" line panicked with slice bounds out of range [2:1]
• line[1] on an a line — a bare "a" line panicked

Both are reachable from a client-supplied WHIP Trickle ICE fragment. Both are now length-checked before indexing. An m= line is additionally required to carry a value, since Marshal drops the m= line when info == "" and an empty media info therefore broke Marshal/Unmarshal symmetry.

The rest of the function was already safe — delimIndex comes from strings.Index, and the bundleIDs[1] access is behind a len > 1 check.

Also in the file

• GetIP — nil deref. ConnectionInformation.Address is a *sdp.Address and can be nil for a c= line the parser accepted without an address; only ConnectionInformation itself was checked.
• PatchICECredentialAndCandidatesIntoSDP — mutation while ranging. The candidate cleanup did md.Attributes = append(md.Attributes[:idx], md.Attributes[idx+1:]...) while ranging over the same slice by index. No panic (the reslices stay within cap), but range captured the original length, so after the first removal every index is stale: consecutive candidates get skipped and stale trailing elements get re-read. Real consequence is old candidates surviving an ICE restart. Replaced with an in-place filter.
• s.media.ice nil derefs in ExtractICECredential and PatchICECredentialAndCandidatesIntoSDP, where a sibling check for it already existed. Defensive — every current construction path sets ice.

Left alone: ExtractFingerprint, ExtractStreamID, GetMediaStreamTrack, GetSimulcastRids, GetBundleMid — their slice accesses are already behind explicit length checks.

One deliberate non-change: "a=group:BUNDLE" with no mids still parses without error (the bundle-mid check is skipped when there is only one token). That is a validation gap rather than a memory-safety one, and it matches GetBundleMid's lenient behavior.

Tests

• TestSDPFragmentUnmarshalMalformed — table of truncated/malformed fragments that must error rather than panic.
• FuzzSDPFragmentUnmarshal — fuzzes Unmarshal, then exercises Mid/Candidates/ExtractICECredential/Marshal on anything that parsed, and asserts the marshalled output re-parses to an equal fragment.

go test ./sdp/ passes; 30s / 12.2M fuzz execs clean. The two panics were confirmed against the pre-fix code.

🤖 Generated with Claude Code

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🦋 Changeset detected

Latest commit: 09cccd3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
github.com/livekit/protocol	Patch
@livekit/protocol	Patch

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