pulse-server: account for underrun

Not quite sure, it needs more investigation but It seems the time
needs to increase when underrunning.

See #510

src/modules/module-protocol-pulse/pulse-server.c

@@ -1458,6 +1458,7 @@ static void stream_process(void *data)
 				pw_stream_flush(stream->stream, true);
 			} else {
 				pd.underrun_for = size;
+				pd.playing_for = size;
 				pd.underrun = true;
 			}
 		} else {