fix crash when starting selection outside grid

src/hexgame.cpp

@@ -116,7 +116,7 @@ void
 updateHexFill(int32 x, int32 y)
 {
 	hex_info *hxi = getSingleHex(x, y);
-	if (hxi && (hxi != g_game_state->current_hex))
+	if (hxi && (hxi != g_game_state->current_hex) && g_game_state->start_hex)
 	{
 		g_game_state->current_hex = hxi;
 		int l = hex_distance(g_game_state->start_hex->hex, g_game_state->current_hex->hex);
@@ -150,7 +150,7 @@ void
 updateHexLineDraw(int32 x, int32 y)
 {
 	hex_info *hxi = getSingleHex(x, y);
-	if (hxi && (hxi != g_game_state->current_hex))
+	if (hxi && (hxi != g_game_state->current_hex) && g_game_state->start_hex)
 	{
 		g_game_state->current_hex = hxi;
 		vector<Hex> hexLine = hex_linedraw(g_game_state->start_hex->hex, hxi->hex);
@@ -189,7 +189,7 @@ void
 updateHexConeFill(int32 x, int32 y)
 {
 	hex_info *hxi = getSingleHex(x, y);
-	if (hxi && (hxi != g_game_state->current_hex))
+	if (hxi && (hxi != g_game_state->current_hex) && g_game_state->start_hex)
 	{
 		g_game_state->current_hex = hxi;