HPCC-8976 - Fix suspect code in CSocket::write_multiple

A counter tracking how much left to send was incorrectly
reduced. When it hit zero it should have wrote last write and exited.
But due to a typo' it was not being reduced as intended.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4111 from jakesmith/hpcc-8976

HPCC-8976 - Fix suspect code in CSocket::write_multiple

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>