ImageMagick affected by divide-by-zero in ThumbnailImage via montage -geometry ":" leads to crash

Summary Passing a geometry string containing only a colon ":" to montage -geometry leads GetGeometry to set width/height to 0. Later, ThumbnailImage divides by these zero dimensions, triggering a crash SIGFPE/abort, resulting in a denial of service. Details Root Cause 1. montage -geometry ":"...