Do not duplicate the buffer if using external data.

Copying fBlockList from orig.GetEND() is currect in this case: orig has only one TMemBlock,
and fExternalData->data() and size() will be the same for any reference to that data.

io/io/src/TMemFile.cxx

@@ -194,8 +194,10 @@ TMemFile::TMemFile(const TMemFile &orig) :
    fD = orig.fD; // not really used, so it is okay to have the same value.
    fWritable = orig.fWritable;
 
-   // We intentionally allocated just one big buffer for this object.
-   orig.CopyTo(fBlockList.fBuffer,fSize);
+   if (!fExternalData) {
+      // We intentionally allocated just one big buffer for this object.
+      orig.CopyTo(fBlockList.fBuffer,fSize);
+   }
 
    Init(!NeedsExistingFile(optmode)); // A copy is
 }