ffmpeg: Don't trigger url_interrupt_cb on the first signal

Currently, the url_interrupt_cb callback will abort all IO
after the first received signal. This makes the output files
from e.g. the mov muxer to be unreadable if the transcode is
aborted with ctrl+c.

After this patch, the first signal cleanly breaks out of
the transcoding loop, but won't forcibly abort all IO.
After the second signal is received, the url_interrupt_cb
callback will abort all IO.

Signed-off-by: Martin Storsjö <martin@martin.st>