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.