Crash during tests after assertion failed in Undo save

[c3d]

While saving for Undo, some peculiar crash, as if the stack had been reset above the Undo region.

doc/commands/comparisons.md:99:## >
help/db50x.md:8234:  Test #81: examples: On-line help examples
help/db50x.md:8234:  81:067.001:  >
help/db50x.md:8234:    Expected output matching [{ True False True True True False }], got [{ False True False False False False }] instead, [True False True True True False }] differs from [False True False False False False }]
| 81: 068 744.2:  ≥                                                      [PASS]
| 81: 069 753.7:  Copy                                                   [PASS]
| 81: 070 758.1:  Assignments with the solver                            [PASS]
| 81: 071 764.7:  Assignments with the solver                            [PASS]
| 81: 072 770.8:  Assignments with the solver                            [PASS]
| 81: 073 774.3:  DrawText                                               [PASS]
| 81: 074 779.9:  ToGrob                                                 [PASS]
| 81: 075 783.9:  GXor                                                   [PASS]
| 81: 076 800.7:  GOr                                                    [PASS]
| 81: 077 810.1:  GAnd                                                   [474834398 5347.002263] assert_error: Assertion failed: ns + (Undo - Stack) < HighMem
[474834399 5347.002289] assert_error: Assertion failed: Stack + depth() < HighMem
=================================================================
==44105==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x633000035890 at pc 0x000106b42da8 bp 0x00016d5de790 sp 0x00016d5ddf40
READ of size 8 at 0x633000035890 thread T9
    #0 0x106b42da4 in memcpy+0x3fc (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x52da4)
    #1 0x102ee2d54 in runtime::save() runtime.cc:1180
    #2 0x1030cf778 in user_interface::end_edit() user_interface.cc:403
    #3 0x1030e2304 in user_interface::do_enter() user_interface.cc:5662
    #4 0x1030d3384 in user_interface::handle_editing(int) user_interface.cc:4688
    #5 0x1030d0a70 in user_interface::key(int, bool, bool) user_interface.cc:603
    #6 0x102daeaf8 in handle_key(int, bool, bool) main.cc:170
    #7 0x102dae7d4 in program_main main.cc:456
    #8 0x102d2feec in RPLThread::run() sim-rpl.cpp:62
    #9 0x1066b2b50 in QThreadPrivate::start(void*)+0x14c (QtCore:arm64+0x19eb50)
    #10 0x106b41858 in asan_thread_start(void*)+0x40 (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x51858)
    #11 0x1841bb2e0 in _pthread_start+0x84 (libsystem_pthread.dylib:arm64e+0x72e0)
    #12 0x1841b60f8 in thread_start+0x4 (libsystem_pthread.dylib:arm64e+0x20f8)