SIGSEGV in Waveform::get_duration() when no waveform is loaded

[nbeaver]

Steps to reproduce:

• File -> New

• Edit -> "Insert After" so there is at least one subtitle.

• Video -> Open.

• Load a video file; which one doesn't matter.

• View -> Waveform. (But don't load an actual waveform; leave it blank.)

• Press play on the video file.

• Click repeatedly on the blank waveform pane.

• Get a segfault. (Sometimes it only takes a few seconds, sometimes it takes longer. Usually 30 mouse clicks is enough.)

This appears to occur because of the waveform duration calculation whenever the waveform is clicked.

Note that the waveform must be blank for this to occur; I have not observed a segfault when a waveform is loaded.

Attached are full stacktraces and Valgrind output.

Reproducible in subtitleeditor version 0.54.0-2 (Ubuntu 18.04) and latest git revision (4c215f4cff4483c44361a2f1d45efc4c6670787f).

valgrind_memcheck.txt

ThreadStacktrace.txt