Fix infinite loop in SFTP file reading

[spencerjanssen]

I ran into this issue working with an SFTP server which always reports an inaccurate file size. However, I think this code is problematic in other scenarios such as the file being truncated mid-read.

[portnov]

Could you please elaborate a bit what the patch does and why. I currently do not have some test suite for this code, so I have to rely on basic logic understanding :)

[spencerjanssen]

So the old code:

1. Gets the reported size of the file, N
2. Reads chunks of the file, in a loop, terminating once N bytes have been read

The problem is that: the size of the file may change in the course of our loop. If the file is truncated the code gets stuck in an infinite loop where each read gets 0 bytes and we never reach N.

The new code doesn't rely on the reported size of the file. Instead, it reads chunks in a loop terminating once sftp_read returns 0 bytes -- which is the way libssh2 signals end of file.