Close IFormatReader copy if not to be used

The generated copy from Memoizer's loadReader logic
was in some cases not being used. When the null was
returned, a RandomAccessFile held by the reader was
left open.

Attn: @zeb @manics

With this commit, a long-running CLI or insight import (say 1000 PNGs or TIFFs) should not lead to constantly increasing file handles on the server. Testing with 1000 .fake files did *not* show the error.