output/PipeWire: add nullptr check to SetVolume()

If the PipeWire output has not yet been enabled and no thread_loop has
been created yet, a nullptr dereference in SetVolume() was possible
because nullptr was passed to pw_thread_loop_lock().

src/output/plugins/PipeWireOutputPlugin.cxx

@@ -312,6 +312,14 @@ PipeWireOutput::PipeWireOutput(const ConfigBlock &block)
 void
 PipeWireOutput::SetVolume(float _volume)
 {
+	if (thread_loop == nullptr) {
+		/* the mixer is open (because it is a "global" mixer),
+		   but Enable() on this output has not yet been
+		   called */
+		volume = _volume;
+		return;
+	}
+
 	const PipeWire::ThreadLoopLock lock(thread_loop);
 
 	float newvol = _volume*_volume*_volume;