If an output has already stopped, but its StopRecording function was
called again, then ffmpeg_mux_stop would be called and set stopping to
true. On the next output start, OBS would output 1 frame, see that
stopping is true, and then stop the output.
This was most easily observed using an Output Timer to record prior to
93f5b45be8.
Initialize stopping to false with the other state flags to ensure that
the output has a clean starting state.
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