Blender Git Loki

Prevent waveform drawing from continually retrying bad files

If sound_read_waveform() cannot read the file (i.e. info.length is
zero), set the sound's waveform to a valid waveform of zero
length. This indicates that reading the waveform is done so that it
doesn't get tried over and over again.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D988