Plugins (Blursk): Fix double free when Blursk is loaded multiple times.

[kaixiong]

Blursk crashes in __blursk_cleanup when the plugin is loaded and unloaded multiple times. This can occur when lv-tool cycles through actors and completes a full rotation.

The bug is due to the sharing and re-use of the variable config across instances.

config contains multiple strings allocated during plugin instance initialization in config_default():

if(conf->color_style)
    visual_mem_free(conf->color_style);
conf->color_style = visual_strdup(config_default_color_style);
if(conf->signal_color)
    visual_mem_free(conf->signal_color);
conf->signal_color = visual_strdup(config_default_signal_color);

These strings are then freed in __blursk_cleanup() when the plugin instance is cleaned up.

visual_mem_free(config.color_style);
visual_mem_free(config.signal_color);

After freeing the strings in the first instance, these pointer values remain unchanged and the next run of config_default() crashes the program.

The solution is to unshare the config struct but I ran into a header inclusion cycle with my attempt. Breaking the cycle requires a fair amount of refactoring, so I decided to instead zero out the config struct to complete the clean-up.

We can come back to this after a more thorough review of the plugin system.