[build2] Bug Report: Stack buffer overrun when downgrading a dependency.

[Karen Arutyunov]

>> Assertion failed: !user_selection () || (keep_out == p.keep_out &&
>> config_vars == p.config_vars), file
>> E:\tools\build2\build2-toolchain-0.9-a.0\bpkg-0.9.0-a.0.20181119143757.859a34416d0a\bpkg\pkg-build.cxx, 
>>
>> line 409
>> error: process bpkg terminated abnormally: stack buffer overrun
> 
> Reproduces for me. I will look into it.

Fixed bpkg is staged.