Don't allow framerate to be set to 0

[dideler]

Setting the framerate of the Video Passthrough to 0 doesn't make sense and it crashes Freeseer.

Traceback (most recent call last):
  File "/home/dennis/github/freeseer/src/freeseer/frontend/record/record.py", line 331, in standby
    if self.load_backend():
  File "/home/dennis/github/freeseer/src/freeseer/frontend/record/record.py", line 418, in load_backend
    if self.controller.load_backend(presentation):
  File "/home/dennis/github/freeseer/src/freeseer/frontend/record/RecordingController.py", line 61, in load_backend
    if self.media.load_backend(presentation):
  File "/home/dennis/github/freeseer/src/freeseer/framework/multimedia.py", line 270, in load_backend
    if not self.load_videomixer(videomixer, videomixer_inputs):
  File "/home/dennis/github/freeseer/src/freeseer/framework/multimedia.py", line 352, in load_videomixer
    self.videomixer.link(self.video_tee)
gst.LinkError: failed to link bin5 with video_tee