Small improvement to copy when dealing with a device (fixes a warning from the drive_cache on the findnext).


git-svn-id: http://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@3874 ed541006-0bf7-43e9-8c4d-6fc63c346d47
Reading from NUL changed to report 0 bytes read. Prevents infinite read loops on NUL.

git-svn-id: http://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@3875 ed541006-0bf7-43e9-8c4d-6fc63c346d47