mdbx++: rework/fix move-assignment operators for "managed" classes.

Replaces #270 and previous commit.
Fixed a half of erigontech/silkworm#575.

mdbx.h++

@@ -3453,7 +3453,15 @@ public:
   void close(bool dont_sync = false);
 
   env_managed(env_managed &&) = default;
-  using inherited::operator=;
+  env_managed &operator=(env_managed &&other) {
+    if (MDBX_UNLIKELY(handle_))
+      MDBX_CXX20_UNLIKELY {
+        assert(handle_ != other.handle_);
+        close();
+      }
+    inherited::operator=(std::move(other));
+    return *this;
+  }
   env_managed(const env_managed &) = delete;
   env_managed &operator=(const env_managed &) = delete;
   virtual ~env_managed() noexcept;
@@ -3744,7 +3752,15 @@ class LIBMDBX_API_TYPE txn_managed : public txn {
 public:
   MDBX_CXX11_CONSTEXPR txn_managed() noexcept = default;
   txn_managed(txn_managed &&) = default;
-  using inherited::operator=;
+  txn_managed &operator=(txn_managed &&other) {
+    if (MDBX_UNLIKELY(handle_))
+      MDBX_CXX20_UNLIKELY {
+        assert(handle_ != other.handle_);
+        abort();
+      }
+    inherited::operator=(std::move(other));
+    return *this;
+  }
   txn_managed(const txn_managed &) = delete;
   txn_managed &operator=(const txn_managed &) = delete;
   ~txn_managed() noexcept;
@@ -3939,7 +3955,16 @@ public:
   void close();
 
   cursor_managed(cursor_managed &&) = default;
-  using inherited::operator=;
+  cursor_managed &operator=(cursor_managed &&other) {
+    if (MDBX_UNLIKELY(handle_))
+      MDBX_CXX20_UNLIKELY {
+        assert(handle_ != other.handle_);
+        close();
+      }
+    inherited::operator=(std::move(other));
+    return *this;
+  }
+
   cursor_managed(const cursor_managed &) = delete;
   cursor_managed &operator=(const cursor_managed &) = delete;
   ~cursor_managed() noexcept { ::mdbx_cursor_close(handle_); }