fix: NULL deref in meta_x11_display_logical_monitor_to_xinerama_index

[appositeit]

Summary

Adds a NULL check for logical_monitor_data in meta_x11_display_logical_monitor_to_xinerama_index() to prevent a segfault that crashes Cinnamon on NVIDIA multi-monitor setups.

Root Cause

meta_x11_display_ensure_xinerama_indices() only attaches xinerama qdata to logical monitors whose geometry exactly matches a Xinerama screen entry (comparing rect.x/y/width/height). With NVIDIA drivers and multiple monitors, after sleep/wake or monitor power cycles, the Xinerama geometry can become stale or mismatched. When a logical monitor has no matching Xinerama entry, get_x11_display_logical_monitor_data() returns NULL and the subsequent dereference logical_monitor_data->xinerama_index segfaults.

The sibling function meta_x11_display_xinerama_index_to_logical_monitor already handles the NULL case correctly.

Crash Signature

cinnamon[PID]: segfault at 0 ip <addr> in libmuffin.so.0.0.0[...]

• Signal 11 (SIGSEGV), NULL pointer dereference at meta_display_logical_index_to_xinerama_index+94
• rax=0x0 (NULL from g_object_get_qdata)
• Call chain: JS → libcjs → libffi → meta_display_logical_index_to_xinerama_index → meta_x11_display_logical_monitor_to_xinerama_index

Environment

• NVIDIA RTX 3080, dual 3440x1440 monitors on DisplayPort
• Ubuntu 25.10, Cinnamon 6.4.12, muffin 6.4.1
• Reproducible: crash occurs within seconds of cinnamon --replace after X session corruption from prior crash

Testing

• Built patched libmuffin.so.0.0.0 from source (meson release build)
• Replaced system library and verified Cinnamon runs stable (28s+ with no crash, vs immediate crash without fix)
• Disassembly confirms NULL check generates test %rax,%rax; je → return -1 path

Fixes #671

🤖 Generated with Claude Code

[appositeit]

Closing in favour of adding this fix to #805, which already addresses related NULL dereference paths. Credit to @gurvesh who independently identified this same issue in #805's comments.