Reapply 782630b minus the whitespace changes

Author: krlmlr

inst/include/cpp11/external_pointer.hpp

@@ -68,9 +68,15 @@ class external_pointer {
     data_ = safe[Rf_shallow_duplicate](rhs.data_);
   }
 
-  external_pointer(external_pointer&& rhs) { reset(rhs.release()); }
+  external_pointer(external_pointer&& rhs) {
+    data_ = rhs.data_;
+    rhs.data_ = R_NilValue;
+  }
 
-  external_pointer& operator=(external_pointer&& rhs) noexcept { reset(rhs.release()); }
+  external_pointer& operator=(external_pointer&& rhs) noexcept {
+    data_ = rhs.data_;
+    rhs.data_ = R_NilValue;
+  }
 
   external_pointer& operator=(std::nullptr_t) noexcept { reset(); };