Library incorrectly assumes 'stat' result to contain a value for 'size'

[hrimhari]

The SFTP protocol allows a server to choose what information to return to a 'stat' command.

When talking to a SSH Tectia Server 5.5 for IBM z/OS and attempting to get a dataset, the server will not return the 'size'. This causes the operation to fail here:

buffer.js:275
  throw new TypeError(kFromErrorMsg);
  ^

TypeError: First argument must be a string, Buffer, ArrayBuffer, Array, or array-like object.
    at fromObject (buffer.js:275:9)
    at Function.Buffer.from (buffer.js:107:10)
    at new Buffer (buffer.js:86:17)
    at tryStat (/Users/fcarasso/Dev/cdr-merger/node_modules/ssh2-streams/lib/sftp.js:1246:16)
    at SFTPStream._transform (/Users/fcarasso/Dev/cdr-merger/node_modules/ssh2-streams/lib/sftp.js:496:15)
    at SFTPStream.Transform._read (_stream_transform.js:167:10)
(...)

I could quickly work around the issue by replacing:

 1238:     size = st.size;

with

1238:      size = st.size || 0;

[hrimhari]

Pull request: #71

[hrimhari]

@mscdex bump