fix state transition bug in PluginAudioThread's Drop implementation

[johnmatter]

before

1. compare_exchange switches state from Processing to Activated
2. Drop calls stop_processing, which contains the following assertion which will fail:

assert_plugin_state_eq!(self, PluginStatus::Processing);

after

IIUC this now has the intended behavior, which is:

• If Processing, then call stop_processing (which transitions to Activated)
• If Activated, do nothing
• Otherwise, panic