Merge branch 'master' into jgrpp

# Conflicts:
#	src/ai/ai_gui.cpp
#	src/company_cmd.cpp
#	src/lang/estonian.txt
#	src/lang/german.txt
#	src/lang/hungarian.txt
#	src/lang/korean.txt
#	src/lang/slovak.txt
#	src/main_gui.cpp
#	src/video/sdl2_v.cpp

src/town_cmd.cpp

@@ -1427,6 +1427,7 @@ static bool GrowTownWithTunnel(const Town* t, const TileIndex tile, const DiagDi
 		/* Only tunnel under a mountain if the slope is continuous for at least 4 tiles. We want tunneling to be a last resort for large hills. */
 		TileIndex slope_tile = tile;
 		for (uint8 tiles = 0; tiles < 4; tiles++) {
+			if (!IsValidTile(slope_tile)) return false;
 			slope = GetTileSlope(slope_tile);
 			if (slope != InclinedSlope(tunnel_dir) && !IsSteepSlope(slope) && !IsSlopeWithOneCornerRaised(slope)) return false;
 			slope_tile += delta;