[release/8.0] Fix si.ibAllocated in FrozenObjectHeapManager (#92052)

* Fix si.ibAllocated in FrozenObjectHeapManager

* Add an assert

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Co-authored-by: NEgorBo <egorbo@gmail.com>

src/coreclr/vm/frozenobjectheap.cpp

@@ -180,10 +180,12 @@ void FrozenObjectSegment::RegisterOrUpdate(uint8_t* current, size_t sizeCommited
         segment_info si;
         si.pvMem = m_pStart;
         si.ibFirstObject = sizeof(ObjHeader);
-        si.ibAllocated = (size_t)current;
+        si.ibAllocated = (size_t)current - (size_t)si.pvMem;
         si.ibCommit = sizeCommited;
         si.ibReserved = m_Size;
 
+        assert((size_t)current >= (size_t)si.pvMem);
+
         // NOTE: RegisterFrozenSegment may take a GC lock inside.
         m_SegmentHandle = GCHeapUtilities::GetGCHeap()->RegisterFrozenSegment(&si);
         if (m_SegmentHandle == nullptr)