AVRO-1238. C: EOF detection in avro_file_reader_read_value

This function now returns the builtin `EOF` constant when end-of-file is
detected.  `0` still indicates a successful read, and other results indicate
errors.

Contributed by Michael Cooper.


git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1486051 13f79535-47bb-0310-9956-ffa450edef68