Fix nullptr dereference during sort.

Add new unit test that will crash without this fix.

Bug: 78900050

Test: Ran unit tests.
Change-Id: I73e1b89e965a7b399822c3a6f25cbc70d2d355e2

1 files changed, 4 insertions, 2 deletions

diff --git a/libc/malloc_debug/PointerData.cpp b/libc/malloc_debug/PointerData.cpp
index f811a5eb4..85139e607 100644
--- a/libc/malloc_debug/PointerData.cpp
+++ b/libc/malloc_debug/PointerData.cpp
@@ -392,10 +392,12 @@ void PointerData::GetList(std::vector<ListInfoType>* list, bool only_with_backtr
     FrameInfoType* b_frame = b.frame_info;
     if (a_frame == nullptr && b_frame != nullptr) {
       return false;
-    }
-    if (a_frame != nullptr && b_frame == nullptr) {
+    } else if (a_frame != nullptr && b_frame == nullptr) {
       return true;
+    } else if (a_frame == nullptr && b_frame == nullptr) {
+      return a.pointer < b.pointer;
     }
+
     // Put the pointers with longest backtrace first.
     if (a_frame->frames.size() != b_frame->frames.size()) {
       return a_frame->frames.size() > b_frame->frames.size();