Fix use after free bug in LinesBucketQueue::emplace_back_bucket

I found a use after free bug in LinesBucketQueue::emplace_back_bucket. This was found by enabling address sanitizer.
The LinesBucketQueue class has two related members:
std::vector<LinesBucket> line_buckets;
std::priority_queue<LinesBucket *, std::vector<LinesBucket *>, LinesBucketPtrComp> line_bucket_ptr_queue;
line_bucket_ptr_queue holds pointers into line_buckets. However, since items are inserted into line_buckets one at a time, existing pointers that were stored inside line_bucket_ptr_queue become invalid. Specifically:
void LinesBucketQueue::emplace_back_bucket(ExtrusionLayers &&els, const void *objPtr, Point offset)
{
    auto oldSize = line_buckets.capacity();
    line_buckets.emplace_back(std::move(els), objPtr, offset); <--- Causes a reallocation, making previous pointers invalid
    line_bucket_ptr_queue.push(&line_buckets.back()); <-- priority queue compares against old, now invalid pointers
    ...

The proposed fix is to calculate the required number of entries in ConflictChecker::find_inter_of_lines_in_diff_objs, and then calling line_buckets.reserve(count). This ensures that sufficient buffer is allocated up front and the pointers are stable as items are added.

src/libslic3r/GCode/ConflictChecker.cpp

@@ -102,6 +102,11 @@ void LinesBucketQueue::emplace_back_bucket(ExtrusionLayers &&els, const void *ob
     }
 }
 
+void LinesBucketQueue::reserve(size_t count)
+{
+    line_buckets.reserve(count);
+}
+
 // remove lowest and get the current bottom z
 float LinesBucketQueue::getCurrBottomZ()
 {
@@ -218,6 +223,14 @@ ConflictResultOpt ConflictChecker::find_inter_of_lines_in_diff_objs(PrintObjectP
 {
     if (objs.size() <= 1 && !wtdptr) { return {}; }
     LinesBucketQueue conflictQueue;
+
+    // Calculate the number of required entries in the conflict queue
+    // One slot is used for the FakeWipeTower and 2 slots for each object
+    size_t requiredCount = 2*objs.size() + (wtdptr.has_value() ? 1 : 0);
+    // Reserve the required count to guarantee that pointers inside LinesBucketQueue::line_buckets
+    // are stable while we append the entries
+    conflictQueue.reserve(requiredCount);
+
     if (wtdptr.has_value()) { // wipe tower at 0 by default
         auto            wtpaths = wtdptr.value()->getFakeExtrusionPathsFromWipeTower();
         ExtrusionLayers wtels;

src/libslic3r/GCode/ConflictChecker.hpp

@@ -114,6 +114,7 @@ public:
 
 public:
     void        emplace_back_bucket(ExtrusionLayers &&els, const void *objPtr, Point offset);
+    void        reserve(size_t count);
     bool        valid() const { return line_bucket_ptr_queue.empty() == false; }
     float       getCurrBottomZ();
     LineWithIDs getCurLines() const;