Fixing inst bug on redefinition.
Fixing inst bug on redefinition.

* after a previous reworking the bus was incorrectly getting
changed when an instrument was redefined... fixed.

la3lma/overtone,chunseoklee/overtone,mcanthony/overtone,ethancrawford/overtone,pje/overtone,brunchboy/overtone,craftybones/overtone,Widea/overtone,rosejn/overtone