Fix null riley crash in Finalize() for volumes and volume filters.

_riley can get nulled out from renderer teardown before Finalize()
completes, so we have to guard against that. See also 2398584.

(Internal change: 2403787)

Author: ohjay

third_party/renderman/plugin/hdPrman/volume.cpp

@@ -209,8 +209,10 @@ HdPrman_Volume::Finalize(HdRenderParam *renderParam)
     if (!_volumeFilterIds.empty()) {
         auto* param = static_cast<HdPrman_RenderParam*>(renderParam);
         riley::Riley* riley = param->AcquireRiley();
-        for (const auto& filterId : _volumeFilterIds) {
-            riley->DeleteVolumeFilter(filterId);
+        if (riley) {
+            for (const auto& filterId : _volumeFilterIds) {
+                riley->DeleteVolumeFilter(filterId);
+            }
         }
         _volumeFilterIds.clear();
     }

third_party/renderman/plugin/hdPrman/volumeFilter.cpp

@@ -52,7 +52,7 @@ HdPrman_VolumeFilter::Finalize(HdRenderParam *renderParam)
     std::lock_guard<std::mutex> lock(_syncToRileyMutex);
     auto* param = static_cast<HdPrman_RenderParam*>(renderParam);
     riley::Riley *riley = param->AcquireRiley();
-    if (_coordSysId != riley::CoordinateSystemId::InvalidId()) {
+    if (riley && _coordSysId != riley::CoordinateSystemId::InvalidId()) {
         riley->DeleteCoordinateSystem(_coordSysId);
         _coordSysId = riley::CoordinateSystemId::InvalidId();
     }