[C++] Incomplete buffer reads in arrow::io::ReadableFile should exactly truncate returned buffer

[asfimport]

Currently the buffer is only being truncated if it is sufficiently small:

https://github.com/apache/arrow/blob/master/cpp/src/arrow/io/file.cc#L416

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-1094. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #737

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 737
#737