solver3: Handle failed assumption in Pop()

If an Assume() failed, we will have pushed the choices stack,
but not actually pushed anything to the solved stack.

This would cause a segmentation fault trying to read the referenced
entry that doesn't actually exists, namely solved[solved.size()].

Handle this by exiting early as there's nothing to do except
pop the entry back out of the array.

1 files changed, 7 insertions, 0 deletions

diff --git a/apt-pkg/solver3.cc b/apt-pkg/solver3.cc
index b97e3b694..3e2f53a26 100644
--- a/apt-pkg/solver3.cc
+++ b/apt-pkg/solver3.cc
@@ -956,6 +956,13 @@ bool APT::Solver::Pop()
 
    _error->Discard();
 
+   // Assume() actually failed to enqueue anything, abort here
+   if (choices.back() == solved.size())
+   {
+      choices.pop_back();
+      return true;
+   }
+
    assert(choices.back() < solved.size());
    int itemsToUndo = solved.size() - choices.back();
    auto choice = solved[choices.back()].assigned;