[SECURITY] Prevent `rhostname` array overflow

[ghsecuritylab]

This is an automatically generated security fix for a vulnerability detected in your code which is a variant of CVE-2020-8597.

You can read in more detail about this vulnerability in CERT Advisory VU#782301.

The vulnerability occurs because, given that vallen was checked to be less than len, it can never be the case that vallen >= len + sizeof(rhostname). Therefore, rhostname never gets trimmed and the rhostname array may overflow.

While this PR was generated for your project automatically, the supporting analysis was performed and verified by the GitHub Security Lab.

The original finding was reported by Ilja Van Sprundel from IOActive.

The proposed patch was developed by Paul Mackerras (paulusmack) from the Samba project in paulusmack/ppp@8d7970b#diff-b7f5f2404cf3f5c09b1f8ad9364bb340.

The original vulnerability got assigned CVE-2020-8597 which has a CVSS v3.1 Base Score of 9.8/10.

Source

The source code that generated and submitted this PR is based on JLLeitschuh/bulk-security-pr-generator.

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