Fix a dictionary modification error in discard_plugin_preset. (#1468)
Fix a dictionary modification error in discard_plugin_preset. (#1468)

* Fix a dictionary modification error in discard_plugin_preset.

PluginHub.discard_plugin_preset iterates over
PluginHub._active_plugins.keys() to identify loaded plugins to remove.
If it actually tries to remove one, this causes a modification to
_active_plugins, which results in an error.

Copying _active_plugins.keys() before iterating prevents this error.

Fixes bug #1464
schieb/angr,schieb/angr,iamahuman/angr,angr/angr,iamahuman/angr,schieb/angr,iamahuman/angr,angr/angr,angr/angr