Fixed Bug #64565 copy doesn't report failure on partial copy

_php_stream_copy_to_stream_ex, when mmap is possible, doesn't check
if actually written bytes match read bytes.

Fix this (paranoid check) to be consistent with non mmap mode

Seems hard to add a unit test, as this rely on a full filesystem.
fix comment
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  fix comment
  Fixed Bug #64565 copy doesn't report failure on partial copy