Adding some more :ssize_t casts to IO.chpl

The move of miniMD to the release/examples directory caused it to be
tested on 32-bit linux for the first time, and revealed new paths in
the IO module that assumed int(64)s would match ssize_t's.  I took
the cheap approach of downcasting them to try and clean up testing
tonight, and will ask Michael to review after the fact.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21926 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
One more IO.chpl cast

Curious, I ran the entire test/io directory on linux32 to see
what else might be lurking and onyl got four failures, all
due to one line.  This cast corrects those.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21927 3a8e244f-b0f2-452b-bcba-4c88e055c3ca