session_free: wrong variable used for keeping state

If libssh2_session_free is called without the channel being freed
previously by libssh2_channel_free a memory leak could occur.

A mismatch of states variables in session_free() prevent the call to
libssh2_channel_free function. session->state member is used instead of
session->free_state.

It causes a leak of around 600 bytes on every connection on my systems
(Linux, x64 and PPC).

(Debugging done under contract for Accedian Networks)

Fixes #246

Signed-off-by: Brian White <mscdex@mscdex.net>
comp_method_zlib_decomp: handle Z_BUF_ERROR when inflating

When using libssh2 to perform an SFTP file transfer from the "JSCAPE MFT
Server" (http://www.jscape.com) the transfer failed. The default JSCAPE
configuration is to enforce zlib compression on SSH2 sessions so the
session was compressed. The relevant part of the debug trace contained:

 [libssh2] 1.052750 Transport: unhandled zlib error -5
 [libssh2] 1.052750 Failure Event: -29 - decompression failure

The trace comes from comp_method_zlib_decomp() in comp.c. The "unhandled
zlib error -5" is the status returned from the zlib function
inflate(). The -5 status corresponds to "Z_BUF_ERROR".

The inflate() function takes a pointer to a z_stream structure and
"inflates" (decompresses) as much as it can. The relevant fields of the
z_stream structure are:

  next_in - pointer to the input buffer containing compressed data
  avail_in - the number of bytes available at next_in
  next_out - pointer to the output buffer to be filled with uncompressed
             data
  avail_out - how much space available at next_out

To decompress data you set up a z_stream struct with the relevant fields
filled in and pass it to inflate(). On return the fields will have been
updated so next_in and avail_in show how much compressed data is yet to
be processed and next_out and avail_out show how much space is left in
the output buffer.

If the supplied output buffer is too small then on return there will be
compressed data yet to be processed (avail_in != 0) and inflate() will
return Z_OK. In this case the output buffer must be grown, avail_out
updated and inflate() called again.

If the supplied output buffer was big enough then on return the
compressed data will have been exhausted (avail_in == 0) and inflate()
will return Z_OK, so the data has all been uncompressed.

There is a corner case where inflate() makes no progress. That is, there
may be unprocessed compressed data and space available in the output
buffer and yet the function does nothing. In this case inflate() will
return Z_BUF_ERROR. From the zlib documentation and the source code it
is not clear under what circumstances this happens. It could be that it
needs to write multiple bytes (all in one go) from its internal state to
the output buffer before processing the next chunk of input but but
can't because there is not enough space (though my guesses as to the
cause are not really relevant). Recovery from Z_BUF_ERROR is pretty
simple - just grow the output buffer, update avail_out and call
inflate() again.

The comp_method_zlib_decomp() function does not handle the case when
inflate() returns Z_BUF_ERROR. It treats it as a non-recoverable error
and basically aborts the session.

Fixes #240

Signed-off-by: Brian White <mscdex@mscdex.net>
Upgrade zlib to 1.2.7
Fixed LIBSSH2_INT64_T_FORMAT macro.

Usually a format macro should hold the whole format, otherwise
it should be named a prefix. Also fixed usage of this macro in
scp.c for a signed var where it was used as prefix for unsigned.

Signed-off-by: Brian White <mscdex@mscdex.net>