Fix end-iterator dereference when pruning source-to-targets map

[kikou]

Summary

• In HdArnoldRenderDelegate::HasPendingChanges, when handling the _dependencyRemovalQueue, the empty() check and the subsequent _sourceToTargetsMap.erase(sourceIt) ran outside the sourceIt != _sourceToTargetsMap.end() guard.
• If the dependency was already absent from the map, find returned end() and we then dereferenced/erased that end iterator — undefined behavior.
• Moved both calls inside the existing validity check so the prune only happens when the lookup actually found an entry.

Test plan

• Render a scene that triggers dependency changes (e.g., reassign a material/light link), confirm no crash and dirty propagation still works.
• Existing render_delegate testsuite passes.

[Copilot]

Pull request overview

This PR fixes undefined behavior in HdArnoldRenderDelegate::HasPendingChanges by ensuring _sourceToTargetsMap is only accessed/erased when the find() call actually returns a valid iterator, preventing an end() iterator dereference during dependency-pruning.

Changes:

• Move sourceIt->second.empty() and _sourceToTargetsMap.erase(sourceIt) inside the sourceIt != _sourceToTargetsMap.end() guard.
• Prevent potential crashes/UB when a dependency removal event references a source that is no longer present in _sourceToTargetsMap.

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