fix assert side-effect

src/hexgrid.cpp

@@ -140,7 +140,8 @@ hgRemoveHex(hexgrid& hg, v3f hex_coords, render_group* rg_filled, render_group*
 		nb_l.count -= IDX_PER_HEX_LINES;
 		ub_l.count -= IDX_PER_HEX_LINES;
 
-		assert(hg.hex_map.erase(hxi->hex) == 1);
+		size_t erased = hg.hex_map.erase(hxi->hex);
+		assert(erased == 1);
 
 		// NOTE: have to repopulate all the buffers, and re-index
 		uint i = 0;