bugfix. this was a tough one. because the observer was already dequeued
from m_transactions, we need to properly terminate it in some way. usually
we go through the end of the function, where reply() does done(). however
there are some error cases where function may returns sooner. this might be
unlikely to occur in original upstream code (missing 'r' key, missing 'id'),
but with IP enforcing it is not so uncommon. hopefully this will fix #32.