video-converter: Fix crashes in fast-paths when converting interlaced formats with different vertical subsampling

E.g. the following pipelines fail because chroma values after the last
line are read (note: 486 % 4 == 2):

gst-launch-1.0 videotestsrc ! "video/x-raw,interlace-mode=interleaved,width=720,height=486,format=UYVY" ! videoconvert ! "video/x-raw,format=I420" ! fakesink
gst-launch-1.0 videotestsrc ! "video/x-raw,interlace-mode=interleaved,width=720,height=486,format=I420" ! videoconvert ! "video/x-raw,format=UYVY" ! fakesink
gst-launch-1.0 videotestsrc ! "video/x-raw,interlace-mode=interleaved,width=720,height=486,format=I420" ! videoconvert ! "video/x-raw,format=AYUV" ! fakesink