b87937a20e
heap-use-after-free. Marker is deleted (and re-created), when
the tempo-map edit is aborted:
```
#0 0x7f77528ac017 in operator delete(void*) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:160
#1 0x55f81062800d in TempoMarker::~TempoMarker() ../gtk2_ardour/marker.cc:794
#2 0x55f80ffb2fc0 in Editor::reset_tempo_marks() ../gtk2_ardour/editor_tempodisplay.cc:205
#3 0x55f80ffb2b19 in Editor::reset_metric_marks() ../gtk2_ardour/editor_tempodisplay.cc:185
#4 0x55f80ffb49fb in Editor::tempo_map_changed() ../gtk2_ardour/editor_tempodisplay.cc:301
#5 0x55f80ffbdf00 in Editor::abort_tempo_map_edit() ../gtk2_ardour/editor_tempodisplay.cc:850
#6 0x55f80fcf967a in TempoMarkerDrag::finished(_GdkEvent*, bool) ../gtk2_ardour/editor_drag.cc:333
```
Since no movement occurred, the tempo-map was not changed.
however we need to drop the lock and writable thread-pointer...
Please see the Ardour web site at https://ardour.org/ for all documentation..
For information on building ardour:
https://ardour.org/development.html