FIX(client): Prevent endless loop when disconnecting PipeWire stream

Since version 0.3.51, PipeWire has support for buffers with a size of 0
being passed in the process callback. More specifically, this feature
was added in commit 96286fb8b11658a0fdaa61194504a3f9541b25e6 ("resample:
use a -1 buffer size to drain") in PipeWire [1].

Unfortunately, this causes an endless loop, because we try to destroy
the PipeWire stream, but it keeps waiting for the node to be completely
drained, which now never happens because it keeps calling
processCallback() where we feed buffers with a size of 0 to it.

To fix this problem, we stop passing buffers to PipeWire when we are
destroying PipeWireOutput by setting m_ok to false on destruction and
checking for isOk() in the callback.

Fixes #5647

[1] https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/96286fb8b11658a0fdaa61194504a3f9541b25e7

src/mumble/PipeWire.cpp

@@ -291,6 +291,7 @@ void PipeWireEngine::stop() {
 	if (m_loop) {
 		pws->pw_thread_loop_stop(m_thread);
 	}
+	m_ok = false;
 }
 
 pw_buffer *PipeWireEngine::dequeueBuffer() {
@@ -395,6 +396,10 @@ PipeWireOutput::~PipeWireOutput() {
 void PipeWireOutput::processCallback(void *param) {
 	auto pwo = static_cast< PipeWireOutput * >(param);
 
+	if (!pwo->m_engine->isOk()) {
+		return;
+	}
+
 	pw_buffer *buffer = pwo->m_engine->dequeueBuffer();
 	if (!buffer) {
 		return;