x264: fix memory leak
x264: fix memory leak

When the allowed caps are the same of template caps the allowed
caps are not unrefed, leaking memory.

https://bugzilla.gnome.org/show_bug.cgi?id=784060

StreamUtils/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GStreamer/gst-plugins-ugly