[hdsi] Lifetime fix found by asan: record the referenced

dirty expression prior to removing it from the collection tables.

Reported by the testLightLinkingSceneIndex test under asan, as:

=1343067==ERROR: AddressSanitizer: heap-use-after-free on address 0x60c0009bd598 at pc 0x7f2b478c6b23 bp 0x7ffff1bf6350 sp 0x7ffff1bf6340
READ of size 8 at 0x60c0009bd598 thread T0
    #0 0x7f2b478c6b22 in std::vector<SdfPathExpression::Op, std::allocator<SdfPathExpression::Op> >::size() const /usr/lib/gcc/x86_64-redhat-linux/11/../../../../include/c++/11/bits/stl_vector.h:919
    #1 0x7f2b478c6b22 in std::vector<SdfPathExpression::Op, std::allocator<SdfPathExpression::Op> >::vector(std::vector<SdfPathExpression::Op, std::allocator<SdfPathExpression::Op> > const&) /usr/lib/gcc/x86_64-redhat-linux/11/../../../../include/c++/11/bits/stl_vector.h:555
    #2 0x7f2b478c6b22 in SdfPathExpression::SdfPathExpression(SdfPathExpression const&) /depts/tools/build_archive/dev/builds/2379295/inst/fedora-gcc64-opt-asan/pxr/include/pxr/usd/sdf/pathExpression.h:54
    #3 0x7f2b478cf89d in std::pair<SdfPathExpression, std::optional<std::pair<SdfPath, TfToken> > >::pair<std::pair<SdfPath, TfToken> const&, true>(SdfPathExpression const&, std::pair<SdfPath, TfToken> const&) /usr/lib/gcc/x86_64-redhat-linux/11/../../../../include/c++/11/bits/stl_pair.h:337
    #4 0x7f2b478cf89d in HdsiLightLinkingSceneIndex_Impl::_Cache::ProcessCollection(SdfPath const&, TfToken const&, SdfPathExpression const&) imaging/hdsi/lightLinkingSceneIndex.cpp:156

(Internal change: 2379722)

Author: blevin

pxr/imaging/hdsi/lightLinkingSceneIndex.cpp

@@ -148,13 +148,15 @@ struct _Cache
                 return;
             }
 
+            // Mark the old expression as dirty.
+            // Do this prior to removing the collection below, because
+            // that will make the table entry referenced by oldExpr expire.
+            _dirtyState.push_back({oldExpr, collectionId});
+
             // Expression has changed.Remove table entries for the existing 
             // collection and queue invalidation.
             _RemoveCollection(
                 collectionId, _InvalidationType::DirtyTargetsAndCollection);
-
-            _dirtyState.push_back({oldExpr, collectionId});
-
         }
 
         if (IsTrivial(expr)) {