calc-milky-way: Prefer g_auto* macros

g_autofree will automatically free variables when they go out of
scope. g_autoptr will do the same, but using the appropriate freefunc
for the given type, e.g. GDateTime -> g_date_time_unref() or
GObject -> g_object_unref().

This automatic cleanup reduces the risk of memory leaks
and allows getting rid of some lines of code.

src/calculations/calculations_milky_way.c

@@ -46,7 +46,7 @@ double
                                        double     latitude)
 {
   double siderial_time;
-  double *coordinates_milky_way;
+  g_autofree double *coordinates_milky_way = NULL;
   double *coordinates_horizontal_milky_way;
 
   coordinates_milky_way = picplanner_get_coordinates_rotational_milky_way ();
@@ -55,8 +55,6 @@ double
                                                                                    latitude,
                                                                                    siderial_time);
 
-  g_free (coordinates_milky_way);
-
   return coordinates_horizontal_milky_way;
 }
 
@@ -70,22 +68,19 @@ double
                                              double     longitude,
                                              double     latitude)
 {
-  GDateTime *iteration_time;
-  double *coordinates_milky_way;
   double *array_coordinates_milky_way = malloc (sizeof (double) * 2 * NUM_DATA_POINTS);
 
   for (int i=0; i<NUM_DATA_POINTS; i++)
     {
-      iteration_time = g_date_time_add_minutes (date_time, i*24*60/NUM_DATA_POINTS-12*60);
+      g_autoptr (GDateTime) iteration_time =
+	g_date_time_add_minutes (date_time, i*24*60/NUM_DATA_POINTS-12*60);
+      g_autofree double *coordinates_milky_way =
+	picplanner_get_coordinates_milky_way (iteration_time,
+					      longitude,
+					      latitude);
 
-      coordinates_milky_way = picplanner_get_coordinates_milky_way (iteration_time,
-                                                                    longitude,
-                                                                    latitude);
       array_coordinates_milky_way[2*i] = coordinates_milky_way[0];
       array_coordinates_milky_way[2*i+1] = coordinates_milky_way[1];
-
-      g_free (coordinates_milky_way);
-      g_date_time_unref (iteration_time);
     }
 
   return array_coordinates_milky_way;
@@ -166,4 +161,4 @@ char
   g_date_time_unref (date_time_disturbance);
 
   return char_no_disturbance;
-}
+}