fixing potential OOB window write when unpacking chm files #2306
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Description
This PR fixes a security vulnerability in file third_party/mspack/lzxd.c that was cloned from clam but did not receive the security patch. The original issue was reported and fixed under Cisco-Talos/clamav@a837736.
This PR applies the same patch to eliminate the vulnerability.
References
https://nvd.nist.gov/vuln/detail/cve-2017-6419
Cisco-Talos/clamav@a837736