pg/AsyncConnection: reset `result_handler` on exception
pg/AsyncConnection: reset `result_handler` on exception

The same commit ec604e0d9bf1652c2c1bea6d84cfe7aee2341dd4 got lost in
the merge.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy