Use FREE macro for freeing colors

Long description:

This macro will set the colors to NULL which avoids a double free on
reload when colors that were unconfigured become configured.

fixes #1299

To apply this patch, use:
curl http://cr.i3wm.org/patch/616/raw.patch | git am

@@ -248,8 +248,7 @@ void parse_config_json(char *json) {

 void free_colors(struct xcb_color_strings_t *colors) {

 #define FREE_COLOR(x)        \

     do {                     \

-        if (colors->x)       \

-            free(colors->x); \

+        FREE(colors->x);     \

     } while (0)

     FREE_COLOR(bar_fg);

     FREE_COLOR(bar_bg);