curl_formget: fix FILE * leak

Properly deal with the fact that the last fread() call most probably is
a short read, and when using callbacks in fact all calls can be short
reads. No longer consider a file read done until it returns a 0 from the
read function.

Reported by: Aaron Orenstein
Bug: http://curl.haxx.se/mail/lib-2011-06/0048.html