pipeline: do not return a value in void functions
pipeline: do not return a value in void functions

Fixes the following compile warnings with MSVC:

warning C4098: 'ngli_node_buffer_unref': 'void' function returning a value

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl