Fix missing buffers sent to analyzer

This is a regression in the upgrade to gstreamer1.0.
The gst_buffer_unref() is incorrect as it removes the buffer when it is still needed by the chunker.
Forcing the pointer to be null prevents it from segfaulting but causes it to skip all chunks in the buffer, dropping the framerate and causing a worse case of #4321.
Removing these 2 lines restores original functionality.
unref buffers going to analyser at the right time

This unrefs the buffer after the last chunk is pulled from it.
Merge pull request #4559 from TheUbuntuGuy/master

Fix missing buffers sent to analyzer