omxbufferpool: fix memory leak if used on output port

omxbufferpool: fix memory leak if used on output port

When using GstOMXBufferPool on an output port, it internally uses
a GPtrArray to manage the GstBuffers instead of the default queue
from the GstBufferPool base class.

In this case GstBufferPool::default_free_buffer is not called when
the pool is stopped. Because the queue is empty. So explicitely
call gst_omx_buffer_pool_free_buffer on each buffer contained in
the GPtrArray.

https://bugzilla.gnome.org/show_bug.cgi?id=726337