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

[Klaim - Joël Lamotte]

To reproduce:

git clone https://github.com/Klaim/build2-debug-myapp.git
bdep init -C ../build-myapp
bdep sync plf-colony/5.0.2

Observed:

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

With:

> b --version
build2 0.9.0-a.0.e4c4d8d65ea6
libbutl 0.9.0-a.0.47e147c212dd
host x86_64-microsoft-win32-msvc14.1
Copyright (c) 2014-2018 Code Synthesis Ltd
This is free software released under the MIT license.