sftp_read: advance offset correctly for buffered copies
sftp_read: advance offset correctly for buffered copies

In the case where a read packet has been received from the server, but
the entire contents couldn't be copied to the user-buffer, the data is
instead buffered and copied to the user's buffer in the next invocation
of sftp_read(). When that "extra" copy is made, the 'offset' pointer was
not advanced accordingly.

The biggest impact of this flaw was that the 'already' variable at the
top of the function that figures out how much data "ahead" that has
already been asked for would slowly go more and more out of sync, which
could lead to the file not being read all the way to the end.

This problem was most noticable in cases where the application would
only try to read the exact file size amount, like curl does. In the
examples libssh2 provides the sftp read function is most often called
with a fixed size large buffer and then the bug would not appear as
easily.

This bug was introduced in the SFTP rewrite in 1.2.8.

Bug: http://curl.haxx.se/mail/lib-2011-08/0305.html
     http://www.libssh2.org/mail/libssh2-devel-archive-2011-08/0085.shtml

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