Backport regression fixes for chansrv

Matt Burtrequested to merge
matt335672-guest/xrdp:chansrv-fixes into master

This request is to backport two fixes for regressions introduced into the drive redirection feature in xrdp 0.9.12

These are both simply regressions and didn't happen with version v0.9.11 of XRDP.

These fixes are already part of xrdp 0.9.13, but xrdp 0.9.12 looks like it's going to be part of Ubuntu 20.04 LTS

Upstream bug reports fixed by these patches are:-

Merge request reports