Fix malformed PUBLISH parsing and handle protocol errors safely

[hjianbo]

Thanks tsuki <t5uki@.> to report this issue via email.

Summary

• add strict bounds checks when parsing incoming PUBLISH topic bytes
• reject invalid MQTT 3.1.1 PUBLISH packets with zero-length topic instead of trapping
• align MQTT 5 behavior: allow empty topic only when Topic Alias is present
• on reader parse/protocol errors, disconnect the socket instead of continuing

Why

Malformed packets could trigger a runtime trap during topic slicing (Range requires lowerBound <= upperBound), causing a remote crash. This change converts malformed input handling into safe parse failure plus connection close.

Tests

• swift test --filter FrameTests/testFramePublishRejectsZeroLengthTopic --filter FrameTests/testFramePublishRejectsZeroLengthTopicInMQTT5WithoutAlias --filter FrameTests/testFramePublishAllowsZeroLengthTopicInMQTT5WithAlias --filter CocoaMQTTReaderProtocolErrorTests

Added tests:

• testFramePublishRejectsZeroLengthTopic
• testFramePublishRejectsZeroLengthTopicInMQTT5WithoutAlias
• testFramePublishAllowsZeroLengthTopicInMQTT5WithAlias
• testMalformedPublishDisconnectsSocket
• testUnknownFrameTypeDisconnectsSocket

[Copilot]

Pull request overview

This PR fixes a remote crash vulnerability where malformed MQTT PUBLISH packets with zero-length topics could trigger a Swift runtime trap (Range requires lowerBound <= upperBound) during topic slicing. The fix adds proper bounds validation in FramePublish parsing and converts all parse/protocol errors in CocoaMQTTReader into safe disconnections rather than silent continuation.

Changes:

• Added strict bounds checks and zero-length topic rejection in FramePublish initialization, with MQTT 5.0 allowing empty topics only when a Topic Alias is present
• Changed CocoaMQTTReader.frameReady() to disconnect the socket on any parse or protocol error instead of logging and continuing
• Added comprehensive test coverage for both the frame-level rejection and reader-level disconnect behavior

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Source/FramePublish.swift	Added bounds checks, zero-length topic rejection for MQTT 3.1.1, and empty-topic-without-alias rejection for MQTT 5.0
Source/CocoaMQTTReader.swift	Replaced printError+break with protocolError+return across all frame parse failures; added protocolError() helper that logs and disconnects
CocoaMQTTTests/FrameTests.swift	Added three tests for zero-length topic handling across MQTT 3.1.1 and 5.0
CocoaMQTTTests/CocoaMQTTReaderProtocolErrorTests.swift	New test file with spy-based tests verifying socket disconnect on malformed publish and unknown frame type

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