[mkgmap-dev] Commit r4179: fix old possible error in index which only showed up because of the new assertions added in r4167.

Version mkgmap-r4179 was committed by gerd on Wed, 25 Apr 2018 fix old possible error in index which only showed up because of the new assertions added in r4167. http://www.mkgmap.org.uk/websvn/revision.php?repname=mkgmap&rev=4179