GUI2/Minimap: fixed a few issues with the new drawing impl

First, the gamemap ptr wasn't reset if map creation failed, meaning the last data set was drawn instead.
Secondly, if no valid map data was provided before the canvas_draw_background function was called, you'd
get a nullptr dereference.

This fix also means invalid maps aren't drawn in the Load Game dialog as before, since the canvas will
clear the texture before calling canvas_draw_background, which will now do nothing if map_ is null.

src/gui/widgets/minimap.cpp

@@ -92,6 +92,7 @@ void minimap::set_map_data(const std::string& map_data)
 	try {
 		map_.reset(new gamemap(std::make_shared<terrain_type_data>(*terrain_), map_data_));
 	} catch(incorrect_map_format_error& e) {
+		map_.reset(nullptr);
 		ERR_CF << "Error while loading the map: " << e.message << '\n';
 	}
 
@@ -101,7 +102,9 @@ void minimap::set_map_data(const std::string& map_data)
 
 void minimap::canvas_draw_background(texture& tex)
 {
-	image::render_minimap(tex, *map_);
+	if(map_) {
+		image::render_minimap(tex, *map_);
+	}
 }
 
 const std::string& minimap::get_control_type() const