MEM: just release a refcount on overview datasets when destroying main dataset

Author: rouault

frmts/mem/memdataset.cpp

@@ -907,7 +907,7 @@ CPLErr MEMDataset::IBuildOverviews(const char *pszResampling, int nOverviews,
                     return CE_Failure;
                 }
             }
-            m_apoOverviewDS.emplace_back(std::move(poOvrDS));
+            m_apoOverviewDS.emplace_back(poOvrDS.release());
         }
     }
 
@@ -1089,7 +1089,7 @@ std::unique_ptr<GDALDataset> MEMDataset::Clone(int nScopeFlags,
         for (const auto &poOvrDS : m_apoOverviewDS)
         {
             poNewDS->m_apoOverviewDS.emplace_back(
-                poOvrDS->Clone(nScopeFlags, bCanShareState));
+                poOvrDS->Clone(nScopeFlags, bCanShareState).release());
         }
 
         poNewDS->SetDescription(GetDescription());

frmts/mem/memdataset.h

@@ -52,7 +52,10 @@ class CPL_DLL MEMDataset CPL_NON_FINAL : public GDALDataset
     std::vector<gdal::GCP> m_aoGCPs{};
     OGRSpatialReference m_oGCPSRS{};
 
-    std::vector<std::unique_ptr<GDALDataset>> m_apoOverviewDS{};
+    using GDALDatasetRefCountedPtr =
+        std::unique_ptr<GDALDataset, GDALDatasetUniquePtrReleaser>;
+
+    std::vector<GDALDatasetRefCountedPtr> m_apoOverviewDS{};
 
     struct Private;
     std::unique_ptr<Private> m_poPrivate;