tray: guard pattern and hotspot allocation

[Rtur2003]

Summary

• Added handling for cairo_pattern_status failures in render_sni; on OOM or pattern creation errors, the function now returns early with clear, actionable logs.
• Guarded hotspot and ID allocations to prevent registration when the internal state is invalid.
• Kept the successful (happy) path unchanged; safety improvements apply only to error paths.

Motivation

• Prevent crashes and undefined behavior when Cairo fails to create a pattern or when heap allocations fail.
• Ensure hotspots are not registered with an invalid or incomplete state, improving overall robustness.

Scope / Safety

• Changes are limited to error-handling paths in swaybar/tray/item.c.
• No behavior changes on the successful path; only early returns and logging were added.
• No external dependencies introduced and no behavioral regressions expected.

Testing

• Tests not run (per instructions).
• Suggested verification if needed:
  • meson setup build && ninja -C build
  • Run swaybar with tray icons
  • Simulate low-memory conditions to exercise error paths