Screen/dc1394: Fix crash on 'StopCapture' if framecount < stopAtFramecount

The way stopAtFramecount was updated at 'StopCapture' time wasn't sound.
Fix this, so setting a stopAtFramecount and then calling 'StopCapture'
before that target count is reached doesn't lead to a crash.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.