RDP/settings: avoid divide-by-zero when optimal client resolution is unavailable

[chrilep]

Summary

This PR prevents a divide-by-zero in guac_rdp_parse_args() when client handshake data does not provide a valid optimal_resolution.

Problem

settings->width and settings->height were derived using:

• ... / user->info.optimal_resolution

If optimal_resolution is 0, this can trigger SIGFPE.

Change

• Guard width/height derivation with if (user->info.optimal_resolution > 0)
• Fall back to defaults (RDP_DEFAULT_WIDTH, RDP_DEFAULT_HEIGHT) otherwise

Behavior

• Existing behavior remains unchanged for valid optimal_resolution
• Invalid/absent resolution metadata now degrades safely to defaults instead of crashing

Scope

Focused bugfix only; no protocol or feature changes.

AI Assistance Disclosure

Parts of this contribution (analysis and code drafting) were created with assistance from GitHub Copilot (GPT-5.3-Codex) and then reviewed/refined before submission.

[necouchman]

@chrilep As with #680, you will need a Jira issue and to update PR title and commit messages with that Jira issue.

[clhedrick]

I understand why the project wants things like a contributor agreement, but the other patch is critical our use of Guacamole.I haven't seen this one yet.

Are you just going to ignore these issues?

We're considering recreating the patches and going through the submission process, but that raises questions about possible copyright violations of the originals.

[necouchman]

@clhedrick We do not require contributor agreements in order to accept pull requests. All that's required is:

• Sign up for a Jira account.
• Create a Jira issue.
• Tag the pull request and commit messages with the Jira ticket number.
• Go through the review process to address concerns from the project committers prior to merging the changes.

As far as "ignoring these issues" - I wouldn't say we're ignoring them, but we do require that those who submit the pull requests be willing to work through the above process in order to accept the changes. Occasionally we will take an in-progress pull request, pull in the commits from the request, and finish out the changes. However, that's less common than trying to get people who are interested in the changes to participate in and contribute to the project.