Reset spikeleveltop and spikelevelbottom in mapclass::loadlevel()

Just a small thing, but if you teleported out of a tower with the
top/bottom screen spikes being onscreen (by dying, for example), they
would retract once you went back in to a tower. Small little thing, but
it's a good thing to polish.

desktop_version/src/Map.cpp

@@ -1150,6 +1150,8 @@ void mapclass::loadlevel(int rx, int ry)
 	ypos = 0;
 	oldypos = 0;
 	extrarow = 0;
+	spikeleveltop = 0;
+	spikelevelbottom = 0;
 
 	//Custom stuff for warplines
 	obj.customwarpmode=false;