agent: Fix a potential re-entrancy situation

agent: Fix a potential re-entrancy situation

If we emit signals from component_io_cb() in the middle of a read, it’s
possible that one of those signals will be picked up by the client to
perform another read. A likely candidate (and what was triggering the
re-entrancy here) would be component-state-changed. Since signals are
emitted synchronously, the second read would being inside the first, and
trigger the anti-re-entrancy assertion.