packet: avoid logging uninitialized sequence number

Sai tenmisonrequested to merge
tenmison/libssh-mirror:patch-1 into master

Add a description of the new feature/bug fix. Reference any relevant bugs.

Fix an uninitialized local variable read in ssh_packet_unimplemented().

This was found by DMSAN. A malformed SSH_MSG_UNIMPLEMENTED packet can omit the required 4-byte sequence number. In that case, ssh_buffer_unpack(packet, "d", &seq) fails and leaves seq uninitialized, but the handler still formats seq into the log message.

This change returns after the failed unpack, so malformed SSH_MSG_UNIMPLEMENTED packets are consumed without logging an uninitialized sequence number. Valid packets keep the existing behavior.

No public bug is referenced.

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • Code modified for feature
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated

Reviewer's checklist:

  • Any issues marked for closing are addressed
  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code

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