Fix file transfers failing at 99% over IRC

File transfers over IRC sometimes failed because the sender `close()`d the socket before reading everything from it, resulting in a TCP RST packet being sent, which was interpreted by the receiver as a problem with the connection, sometimes dropping the last few bytes of the file as a result.
This patch reads everything from the socket before closing, thus avoiding the RST packet issue.

Testing Done:
Tested on Windows, Linux, Windows-to-Linux and vice-versa IRC file transfers.

Bugs closed: PIDGIN-15893

Reviewed at https://reviews.imfreedom.org/r/1385/