h264parse: Fix splitting of multiple NALU per buffer

Conversion to byte-stream/nal crashes without that because the
baseparse frame of all NALUs is finished for the first NALU, then
used again for parsing the second NALU. Just that now the buffer
of the frame is already gone. Instead we create temporary frames
for every NALU.
opencv: don't unref NULL caps in finalize

Fixes gst-inspect-1.0 -a and generic states unit test.