[42] session-rdp: Fix check when creation of virtual channel manager fails
WTSOpenServerA() returns INVALID_HANDLE_VALUE, which is not NULL, when the creation of the virtual channel manager fails. Since NULL is an invalid value here too, check against that value too. This is what the sample server implementations in FreeRDP also do.
Fixes: https://errors.ubuntu.com/problem/4b04e96f4f5f92ed74e814927dadeab1fba5fbdd
(cherry picked from commit dbfc75af)