fix: harden screen selection in RvApplication against invalid indices (#1211)

[pjurkas]

Linked issues

Fixes #1211

Summarize your change.

Harden screen selection in RvApplication against invalid QScreen indices and
null pointers so OpenRV no longer crashes (SIGSEGV in QScreen::geometry()) at
startup when multiple displays are connected.

Describe the reason for the change.

Issue #1211 reports a SIGSEGV in Rv::RvApplication::newSessionFromFiles →
QScreen::geometry() on multi-monitor setups (RHEL 9.5). The crash happens
because the existing code dereferences QScreen* values that can be nullptr,
and indexes into QGuiApplication::screens() with values (from saved
preferences or -screen command-line options) that may be out of range or
stale. Under certain GNOME multi-display configurations the primary screen or
the requested screen index is not what the code assumes, and we crash.

Describe what you have tested and on which operating system.

• RHEL 9.5 with multiple displays attached — OpenRV now starts cleanly on
  configurations that previously reproduced the SIGSEGV from [Bug]: Crash in RvApplication::newSessionFromFiles while multiple displays are connected. #1211.
• Single-display setup — no behavior change observed.
• Stale/out-of-range -screen command-line value — falls back gracefully
  instead of crashing.

Add a list of changes, and note any that might need special attention during the review.

src/lib/app/RvCommon/RvApplication.cpp:

• Guard isVirtualDesktop against a null primary screen and null entries in
  QGuiApplication::screens().
• Use QGuiApplication::screenAt() for more robust point-to-screen mapping,
  with a manual geometry fallback when it returns null.
• Validate opts.screen against the current screen count before applying it,
  so stale preferences or out-of-bounds command-line values do not propagate.
• Bounds-check screen indices and null-check QScreen* pointers before
  dereferencing geometry() in the relative-position adjustment path.

Reviewer focus: please double-check the fallback path when
QGuiApplication::screenAt() returns null and the chosen index when
opts.screen is invalid — those are the behavioral edges most likely to
affect existing multi-monitor workflows.

If possible, provide screenshots.

N/A — crash fix.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: pjurkas / name: Pavel Jurkas (a7744cb)

[cedrik-fuoco-adsk]

Could you add curly braces? That style is not followed everywhere, but we are trying to add curly braces whenever we can.

[cedrik-fuoco-adsk]

Thank you @pjurkas! Could you run the pre-commit hook and fix the DCO issue?