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.
See https://github.com/joshmoore/bioformats/commit/b589d05ed11a73de2f676d5308cd559c86c61e38 -- that way at least I don't have to worry about how to wrap the exception, etc.