soundio_disconnect: null out more function pointers
soundio_disconnect: null out more function pointers

tresf/libsoundio,island-org/libsoundio,island-org/libsoundio,davidgranstrom/libsoundio,davidgranstrom/libsoundio,andrewrk/libsoundio,tmatth/libsoundio,tresf/libsoundio,andrewrk/libsoundio,tmatth/libsoundio