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.