[mkgmap-dev] Commit r4278: correct code that checks for existing ovm_*.img,

Version mkgmap-r4278 was committed by gerd on Wed, 13 Feb 2019 correct code that checks for existing ovm_*.img, The old code might have removed an ovm_*.img file in the working directory instead of the output directory when processing non-osm input files. http://www.mkgmap.org.uk/websvn/revision.php?repname=mkgmap&rev=4278