examples: test-appsrc: fix buffer leak

calling signal "push-buffer" on appsrc does not transfer ownership of buffer: unref buffer to avoid memory leak

look into: gstappsrc.c: function gst_app_src_push_buffer_action calls gst_app_src_push_buffer_full (appsrc, buffer, FALSE). last argument to function gst_app_src_push_buffer_full does define whether or not to steal the buffer-reference.

using signal "push-buffer" on appsrc therefore needs gst_buffer_unref when we are done with the buffer.