manual: Fix buffer memory leak in appsrc example
manual: Fix buffer memory leak in appsrc example

g_signal_emit_by_name() is not like gst_app_src_push_buffer() due to reference
counting limitations of signals, it does *not* take ownership of the buffer.

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer