pulse-server: recover for underrun better

If we are underrun, don't update the read pointer and let the write
pointer catch up.
If we don't have enough data to fill a buffer, skip all the available
data and wait for the new request to arrive.

Fixes #1857

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

@@ -1243,8 +1243,10 @@ static void stream_process(void *data)
 			if (stream->attr.prebuf == 0 && !stream->corked) {
 				pd.missing = size;
 				pd.playing_for = size;
-				index += size;
-				pd.read_inc = size;
+				if (avail > 0) {
+					index += avail;
+					pd.read_inc = avail;
+				}
 				spa_ringbuffer_read_update(&stream->ring, index);
 			}
 		} else {